CLASS-BOOK OF GEOLOGY BY ARCHIBALD GEIKIE, F.R.S. DIRECTOR-GENERAL OF THE GEOLOGICAL SURVEY OF THE UNITED KINGDOM, AND DIRECTOR OF THE MUSEUM OF PRACTICAL GEOLOGY, LONDON J FORMERLY MURCHISON PROFESSOR OF GEOLOGY AND'MINERALOGY IN THE UNIVERSITY OF EDINBURGH ILLUSTRATED WITH WOODCUTS OP THR HIVER SIT 7 ILontron MACMILLAN AND CO. AND NEW YORK 1891 First Edition 1886. Second Edition 1890. Reprinted 1891. PREFACE TO THE FIRST EDITION THE present volume completes a series of educational works on Physical Geography and Geology, projected by me many years ago. In the Primer s^ published in 1873, tne m ost elementary facts and principles were presented in such a way as I thought most likely to attract the learner, by stimulating at once his faculties of observation and reflection. The continued sale of large editions of these little books in this country and in America, and the translation of them into most European languages, leads me to believe that the practical methods of instruction adopted in them have been found useful. They were followed in 1877 by my Class-Book of Physical Geography ', in which, upon as far as possible the same line of treatment, the subject was developed with greater breadth and fulness. This volume was meant to be immediately succeeded by a corresponding one on Geology, but pressure of other engage- ments has delayed till now the completion of this plan. So many introductory works on Geology have been written that some apology or explanation seems required from an author who adds to their number. Experience of the practical work of teaching science long ago convinced me that what the young learner primarily needs is a class-book which will awaken vi PREFACE his curiosity and interest. There should be enough of detail to enable him to understand how conclusions are arrived at. All through its chapters he should see how observation, generalisation, and induction go hand in hand in the progress of scientific research. But it should not be overloaded with technical details which, though of the highest importance, cannot be adequately understood until considerable advance has been made in the study. It ought to present a broad, luminous picture of each branch of the subject, necessarily, of course, incomplete, but perfectly correct and intelligible as far as it goes. This picture should be amplified in detail by a skilful teacher. It may, however, so arrest the attention of the learner himself as to lead him to seek, of his own accord, in larger treatises, fuller sources of information. To this ideal standard of a class-book I have striven in some measure to approach . Originally, I purposed that this present volume should be uniform in size with the Class-Book of Physical Geography. But, as the illustrations were in progress, the advantage of adopting a larger page became evident, and with this greater scope and my own enthusiasm for the subject the book has gradually grown into what it now is. With few exceptions, the woodcuts have been drawn and engraved expressly for this volume. Mr. Sharman has kindly made for me most of the drawings of the fossils. The landscape sketches are chiefly from my own note-books. I have to thank Messrs. J. D. Cooper and M. Lacour for the skill with which they have given in wood-engraving the expression of the originals. PREFACE vii In preparing the Second Edition, I have thoroughly revised this Class-book, so as to keep it abreast of the onward progress of Geology. The sale of a large impression, and the numerous communications received from teachers and others, have led to the belief that the book might be made still more useful if printed in such a form as to admit of its being sold at a greatly reduced price. This change has now been effected; but the volume, though diminished in bulk, contains rather more matter than the first edition. Care has been taken to make the Index full and accurate. ysthjune 1890. CONTENTS CHAPTER I PAGE INTRODUCTORY ..... .1 PART I THE MATERIALS FOR THE HISTORY OF THE EARTH CHAPTER II THE INFLUENCE OF THE ATMOSPHERE IN THE CHANGES OF THE EARTH'S SURFACE . . . . .10 CHAPTER III THE INFLUENCE OF RUNNING WATER IN GEOLOGICAL CHANGES, AND HOW IT IS RECORDED . . '. 23 CHAPTER IV THE MEMORIALS LEFT BY LAKES 42 CHAPTER V HOW SPRINGS LEAVE THEIR MARK IN GEOLOGICAL HIS- TORY . . . . . . . . .49 CONTENTS CHAPTER VI PAGE ICE-RECORDS 6 1 CHAPTER VII THE MEMORIALS OF THE PRESENCE OF THE SEA . 70 v CHAPTER VIII HOW PLANTS AND ANIMALS INSCRIBE THEIR RECORDS IN GEOLOGICAL HISTORY 8 I CHAPTER IX THE RECORDS LEFT BY VOLCANOES AND EARTHQUAKES 93 PART II ROCKS, AND HOW THEY TELL THE HISTORY OF THE EARTH CHAPTER X THE MORE IMPORTANT ELEMENTS AND MINERALS OF THE EARTH'S CRUST . . . . . 115 CHAPTER XI THE MORE IMPORTANT ROCKS AND ROCK-STRUCTURES IN THE EARTH'S CRUST , V 139 CONTENTS xi PART III THE STRUCTURE OF THE CRUST OF THE EARTH CHAPTER XII PAGE SEDIMENTARY ROCKS THEIR ORIGINAL STRUCTURES . 171 CHAPTER XIII SEDIMENTARY ROCKS STRUCTURES SUPERINDUCED IN THEM AFTER THEIR FORMATION . . . .185 CHAPTER XIV ERUPTIVE ROCKS AND MINERAL VEINS IN THE ARCHI- TECTURE OF THE EARTH'S CRUST .... 200 CHAPTER XV HOW FOSSILS HAVE BEEN ENTOMBED AND PRESERVED, AND HOW THEY ARE USED IN INVESTIGATING THE STRUCTURE OF THE EARTH'S CRUST, AND IN STUDY- ING GEOLOGICAL HISTORY 212 PART IV THE GEOLOGICAL RECORD OF THE HISTORY OF THE EARTH CHAPTER XVI THE EARLIEST CONDITIONS OF THE GLOBE THE ARCHAEAN PERIODS . . .226 xn CONTENTS CHAPTER XVII PAGE THE PALAEOZOIC PERIODS CAMBRIAN . . .237 CHAPTER XVIII SILURIAN ..... ... 248 CHAPTER XIX DEVONIAN AND OLD RED SANDSTONE . . . .258 CHAPTER XX CARBONIFEROUS 267 CHAPTER XXI PERMIAN 284 CHAPTER XXII THE MESOZOIC PERIODS TRIASSIC . . . .291 CHAPTER XXIII JURASSIC .......... 299 CHAPTER XXIV CRETACEOUS 313 CHAPTER XXV TERTIARY OR CAINOZOIC EOCENE OLIGOCENE . 327 CONTENTS xih CHAPTER XXVI PAGE MIOCENE PLIOCENE 339 CHAPTER XXVII POST -TERTIARY OR QUATERNARY PERIODS PLEISTO- CENE OR POST-PLIOCENE RECENT . . 351 APPENDIX . . . . .369 INDEX . ..... . 385 LIST OF ILLUSTRATIONS FIG. PAGE 1. Weathering of rock, as shown by old masonry. (The "false- bedding " and other original structures of the stone are revealed by weathering) . . . . . . .12 2. Passage of sandstone upwards into soil . . . . .15 3. Passage of granite upwards into soil . . . . . .15 4. Talus-slopes at the foot of a line of cliffs . . . . 19 5. Section of rain-wash or brick-earth ...... 20 6. Sand-dunes 21 7. Erosion of limestone by the solvent action of a peaty stream, Dur- ness, Sutherlandshire ........ 25 8. Pot-holes worn out by the gyration of stones in the bed of a stream 30 9. Grand Canon of the Colorado ....... 33 10. Gullies torn out of the side of a mountain by descending torrents, with cones of detritus at their base . . . . . -35 11. Flat stones in a bank of river-shingle, showing the direction of the current that transported and left them ..... 37 12. Section of alluvium showing direction of currents .... 37 13. River-terraces .......... 38 14. Alluvial terraces on the side of an emptied reservoir ... 43 15. Parallel roads of Glen Roy ........ 44 16. Stages in the filling up of a lake ...... 45 17. Piece of shell-marl containing shells of Limncea peregra . . 46 18. View of Axmouth landslip as it appeared in April 1885 ... 51 19. Section of cavern with stalactites and stalagmite .... 54 20. Section showing successive layers of growth in a stalactite . . 55 21. Travertine with impressions of leaves ...... 58 22. Glaciers and moraines ........ 63 23. Perched blocks scattered over ice-worn surface of rock ... 64 24. Stone -smoothed and striated by glacier-ice ..... 66 xvi LIST OF ILLUSTRATIONS FIG. PAGE 25. Ice-striation on the floor and side of a valley .... 67 26. Buller of Buchan a caldron-shaped cavity or blow-hole worn out of granite by the sea on the coast of Aberdeenshire . . -7* 27. The Stacks of Duncansby, Caithness, a wave-beaten coast-line . 73 28. Section of submarine plain ........ 74 29. Storm -beach ponding back a stream and forming a lake; west coast of Sutherlandshire ........ 77 30. Section of a peat-bog . . . . . . . . .82 31. Diatom -earth from floor of Antarctic Ocean, magnified 300 diameters .......... 84 32. Recent limestone (cockle, etc.) ....... 85 33. Globigerina ooze magnified ........ 86 34. Section of a coral-reef ........ 87 35. Cellular lava with a few of the cells filled up with infiltrated mineral matter (Amygdules) ........ 97 36. Section of a lava-current ........ 98 37. Elongation of cells in direction of flow of a lava-stream . . .99 38. Volcanic block ejected during the deposition of strata in water . 102 39. Volcanoes on lines of fissure ....... 104 40. Outline of a volcanic neck . . . . . . . .106 41. Ground-plan of the structure of the Neck shown in Fig. 40 . .106 42. Section through the same Neck as in Figs. 40 and 41 . . .107 43. Volcanic dykes rising through the bedded tuff of a crater . .109 44. Group of quartz-crystals (Rock-crystal) . . . . .118 45. Calcite (Iceland spar), showing its characteristic rhombohedral cleavage .......... 124 46. Cube, octahedron, dodecahedron . . . . . .125 47. Tetragonal prism and pyramid . . . . . . .125 48. Orthorhombic prism . . . . . . . .125 49. Hexagonal prism, rhombohedron, and scalenohedron . . .126 50. Monoclinic prism. Crystal of Augite . . . . . .126 51. Triclinic prism. Crystal of Albite felspar ..... 126 52. Section of a pebble of chalcedony ...... 128 53. Piece of haematite, showing the nodular external form and the in- ternal crystalline structure . . . . . . .129 54. Octahedral crystals of magnetite in chlorite schist . . . .130 55. Dendritic markings due to arborescent deposit of earthy manganese oxide ........... 131 56. Cavity in a lava, filled with zeolite which has crystallised in long slender needles ......... 132 57. Hornblende crystal . . . . . . . . -133 58. Olivine crystal . . . . . . . . . 133 59. Calcite in the form of " nail-head spar " ..... 134 LIST OF ILLUSTRATIONS xvii FIG. PAGE 60. Calcite in the form of dog-tooth spar . . . . . -135 61. Sphaerosiderite or Clay-ironstone concretion enclosing portion of a fern ........... 136 62. Gypsum crystals . . , . . . . . 137 63. Group of fluor-spar crystals ....... 138 64. Concretions . . . . . . . . . . 140 65. Section of a septarian nodule, with coprolite of a fish as a nucleus . 141 66. Piece of oolite .......... 142 67. Piece of pisolite . . . . . , . , . .142 68. Cavities in quartz containing liquids (magnified) . . . .143 69. Various forms of crystallites (highly magnified) , 144 70. Porphyritic structure . . . . . . . . 145 71. Spherulites and fluxion-structure . . . . . . .145 72. Schistose structure . . . . . . . . -147 73. Brecciated structure volcanic breccia, a rock composed of angular fragments of lava, in a paste of finer volcanic debris . . . 149 74. Conglomerate ....... 150 75. Concretionary forms assumed by Dolomite, Magnesian Limestone, Durham . . . . . . . . . .156 76. Weathered surface of crinoidal limestone ..... 159 77. Group of crystals of felspar, quartz, and mica, from a cavity in the Mourne Mountain granite ....... 164 78. Columnar basalts of the Isle of Staffa, resting upon tuff (to the right is Fingal's Cave) . . . . . .166 79. Section of stratified rock ........ 172 80. Section showing alternation of beds ...... 174 81. False-bedded sandstone ........ 175 82. Ripple-marked surface . . . . . . . .176 83. Cast of a sun-cracked surface preserved in the next succeeding layer of sediment . . . . . . . . .177 84. Rain-prints on fine mud . . . . . . . .178 85. Vertical trees (Sigillaria) in sandstone, Swansea (Logan) . .180 86. Hills formed out of horizontal sedimentary rocks .... 181 87. Section of overlap ......... 182 88. Unconformability . . . . . . . . .183 89. Joints in a stratified rock . . . . . . . .186 90. Dip and Strike 188 91. Clinometer .......... 188 92. Dip, Strike, and Outcrop . . . . . . . .189 93. Inclined strata shown to be parts of curves ..... 190 94. Curved strata (anticlinal fold), near St. Abb's Head . . . 191 95. Curved strata (synclinal fold) near Banff . . . . .192 96. Anticlines and Synclines . . . . . . 193 xviii LIST OF ILLUSTRATIONS FIG. PAGE 97. Section of folded and crumpled strata forming the Grosse Windgalle (10,482 feet), Canton Uri, Switzerland, showing crumpled and inverted strata (after Heim) . . . . . . .193 98. Distortion of fossils by the shearing of rocks . , . 194 99. Curved and cleaved rocks. Coast of Wigtonshire . . . 195 100. Examples of normal Faults . . . . . . 195 101. Sections to show the relations of Plications to reversed Faults . 196 102. Throw of a Fault ......... 197 103. Ordinary unaltered red sandstone, Keeshorn, Ross-shire . .198 104. Sheared red sandstone forming now a micaceous schist, Keeshorn, Ross-shire .......... 198 105. Outline and section of a Boss traversing stratified rocks . . 202 106. Ground-plan of Granite-boss with ring of Contact-Metamorphism 203 107. Intrusive Sheet ......... 204 108. Interstratified or contemporaneous Sheets ..... 205 109. Section to illustrate evidence of contemporaneous volcanic action 205 no. Map of Dykes near Muirkirk, Ayrshire ..... 207 in. Section of a volcanic neck . . 208 112. Section of a mineral vein ........ 209 113. Common Cockle (Cardium edule] . . . . , .216 114. Fragment of crumpled Schist ....... 234 115. A, Fucoid-like impression (Eophyton Linneanum) from Cambrian rocks (I) . ~ ......... 242 116. Oldhamia radiata (natural size), Ireland ..... 243 117. Hydrozoon from the Cambrian rocks . . 243 1 1 8. Cambrian Trilobites ........ 244 119. Cambrian Brachiopod (Lingulella Davisii), natural size . . 246 120. An Upper Silurian sea- weed (Chondrites versimilis], natural size 249 121. Graptolites from Silurian rocks . . . . . . .250 122. Silurian Corals . . . . . . . . , 251 123. Silurian Echinoderms ........ 252 124. Filled-up Burrows or Trails left by a sea- worm on the bed of the Silurian sea (Lumbricaria antiqua, ^) ..... 253 125. Trilobites (Lower and Upper Silurian) ..... 254 126. Silurian Phyllopod Crustacean .... . 255 127. Silurian Brachiopods ........ 255 128. Silurian Lamellibranch . . . . . . . .256 129. Silurian Gasteropod . . . . . . . . .256 130. Silurian Cephalopods . . . . . . , -257 131. Plants of the Devonian period . . . . . . .259 132. Overlapping scales of an Old Red Sandstone fish . . . 260 133. Scale-covered Old Red Sandstone fishes . . . . .261 134. Plate-covered Old Red Sandstone fishes . . . . .261 LIST OF ILLUSTRATIONS xix FIG. PAGE 135. Devonian Eurypterid Crustacean ...... 262 136. Devonian Trilobites 263 137. Devonian Corals ......... 263 138. Devonian Brachiopods ........ 264 139. Devonian Lamellibranch and Cephalopod ..... 265 140. Section of part of the Cape Breton coal-field, showing a succession of buried trees and land-surfaces ...... 270 141. Carboniferous Ferns . ......... 272 142. Carboniferous Lycopod ........ 273 143. Carboniferous Equisetaceous Plants ...... 274 144. Sigillaria with Stigmaria roots ....... 274 145. Cordaites alloidius ......... 275 146. Carboniferous Foraminifer ....... 276 147. Carboniferous Rugose Corals ....... 277 148. Carboniferous Sea-Urchin . . . . , . . . 278 149. Carboniferous Crinoid ........ 278 150. Carboniferous Blastoid ........ 278 151. Carboniferous Trilobite ........ 278 152. Carboniferous Polyzoon ........ 279 153. Carboniferous Brachiopods ....... 280 154. Carboniferous Lamellibranchs ....... 280 155. Carboniferous Gasteropods . . . . . . .281 156. Carboniferous Pteropod . . . . . . . .281 157. Carboniferous Cephalopods ....... 281 158. Carboniferous Fishes . . . . . . . .282 159. Permian Plants ......... 286 1 60. Permian Brachiopods ........ 287 161. Permian Lamellibranchs . . . . . . . .288 162. Permian Ganoid Fish . . . . . . . .288 163. Permian Labyrinthodont ........ 289 164. Triassic Plants ......... 293 165. Triassic Crinoid ......... 294 1 66. Triassic Lamellibranchs ........ 295 167. Tfiassic Cephalopods ........ 295 168. Triassic Lizard ......... 296 169. Triassic Crocodile Scutes . . . . . . . . 296 170. Triassic Marsupial Teeth ..... , 296 171. Jurassic Cycad . ..... .... 300 172. Jurassic reef-building Coral ....... 300 173. Jurassic Crinoid ......... 301 174. Jurassic Sea-urchin . . . . . . . . .301 175. Jurassic Lamellibranchs . ... . . . . . 302 176. Jurassic Ammonites . . . . . . . . . 303 xx LIST OF ILLUSTRATIONS FIG. PAGE 177. Jurassic Belemnite . . . . . \ . . . . . 303 178. Jurassic Crustacean . ..-.-. \ . . . 304 179. Jurassic Fish . - . . . . . . \ . . . 304 1 80. Jurassic Sea-lizard . .-.'. . . . . 305 181. Jurassic Pterosaur, or flying reptile ...... 306 182. Jurassic Bird ... . . ' . . . . . . 307 183. Jurassic Marsupial Teeth and Jaw ....... 308 184. Cretaceous Plants . . . . . . . . -3*5 185. Cretaceous Foraminifera ........ 316 1 86. Cretaceous Sponge . . . . . . , . .316 187. Cretaceous Sea-urchins . . . . . . . .317 188. Cretaceous Lamellibranchs . . . . . . .318 189. Cretaceous Lamellibranchs . . . . . . .318 190. Cretaceous Cephalopods . . . . . . . .319 191. Cretaceous Fish ......... 320 192. Cretaceous Deinosaur ........ 321 193. Eocene Plant . . . . . . . . . . 331 194. Eocene Molluscs . . . . . . . . . 331 195. Eocene Mammal ......... 332 196. Skull of Tinoceras ingens ........ 333 197. Oligocene Molluscs . . . . . . . . -336 198. Miocene Plants ......... 340 199. Mastodon augustidens ........ 341 200. Skull of Deinotherium giganteiim ...... 342 201. Pliocene Plants ......... 346 202. Pliocene Marine Shells . . .. .... . . 347 203. Helladothcrium Du-vernoyi a gigantic animal intermediate in structure between the giraffe and the antelope, Pikermi, Attica ........... 350 204. Pleistocene or Glacial Shells . . . . . . -356 205. Mammoth, from the skeleton in the Muse"e Royal, Brussels . . 357 206. Back view of skull of Musk-sheep, Brick-earth, Crayford, Kent . 357 207. Palaeolithic Implements ... ...... 362 208. Antler of Reindeer found at Bilney Moor, East Dereham, Norfolk 364 209. Neolithic Implements ........ 366 CHAPTER I INTRODUCTORY THE main features of the dry land on which we live seem to remain unchanged from year to year. The valleys and plains familiar to our forefathers are still familiar to us, bearing the same meadows and woodlands, the same hamlets and villages, though generation after generation of men has meanwhile passed away. The hills and mountains now rise along the sky-line as they did long centuries ago, catching as of old the fresh rains of heaven and gathering them into the brooks and rivers which, through unknown ages, have never ceased to flow seawards. So stead- fast do these features appear to stand, and so strong a contrast do they offer to the shortness and changeableness of human life, that they have become typical in our minds of all that is ancient and durable. We speak of the firm earth, of the everlasting hills, of the imperishable mountains, as if, where all else is fleeting and mutable, these forms at least remain unchanged. And yet attentive observation of what takes place from day to day around us shows that the surface of a country is not now exactly as it used to be. We notice various changes of its topo- graphy going on now, which have doubtless been in progress for a long time, and the accumulated effect of which may ultimately transform altogether the character of a landscape. A strong gale, for instance, will level thousands of trees in its pathway, turning a tract of forest or woodland into a bare space, which may become a quaking morass, until perhaps changed into arable ground by the farmer. A flooded river will in a few hours cut away large slices from its banks, and spreading over fields and meadows, will bury many acres of fertile land under a covering of barren sand and shingle. A long-continued, heavy rain, by loosening masses B 2 INTRODUCTORY CHAP. of earth or rock on steep slopes, causes destructive landslips. A hard frost splinters the naked fronts of crags and cliffs, and breaks up bare soil. In short, every shower of rain and gust of wind, if we could only watch them narrowly enough, would be found to have done something towards modifying the surface of the land. Along the sea-margin, too, how ceaseless is the progress of change ! In most places, the waves are cutting away the land, sometimes even at so fast a rate as two or three feet in a year. Here and there, on the other hand, they cast sand and silt ashore so as to increase the breadth of the dry land. These are ordinary everyday causes of alteration, and though singly insignificant enough, their united effect after long centuries cannot but be great. From time to time, however, other less frequent but more powerful influences come into play. In most large regions of the globe, the ground is often convulsed by earth- quakes, many of which leave permanent scars upon the surface of the land. Volcanoes, too, in many countries pour forth streams of molten rock and showers of dust and cinders that bury the surrounding districts and greatly alter their appearance. Turning to the pages of human history, we find there the records of similar changes in bygone times. Lakes, on which our rude forefathers paddled their canoes and built their wattled island-dwellings, have wholly disappeared. Bogs, over whose treacherous surface these early hunters could not follow the chase of red deer or Irish elk, have become meadows and fields. Forests, where they hunted the wild boar, have been turned into grassy pastures. Cities have been entirely destroyed by earthquakes or have been entombed under the piles of ashes discharged from a burning mountain. So great have been the inroads of the sea that, in some instances, the sites of what a few hundred years ago were farms and hamlets, now lie under the sea half a mile or more from the modern shore. Elsewhere the land has gained upon the sea, and the harbours of an earlier time are now several miles distant from the coast-line. But man has naturally kept note only of the more impressive changes ; in other words, of those which had most influence upon his own doings. We may be certain, however, that there have been innumerable minor alterations of the surface of the land within human history, of which no chronicler has made mention, either because they seemed too trivial, or because they took place so imperceptibly as never to be noticed. Fortunately, in many cases, these mutations of the land have written their own memorials, i GEOLOGICAL CHANGES WITNESSED BY MAN 3 which can be as satisfactorily interpreted as the ancient manu- scripts from which our early national history is compiled. In illustration of the character of these natural chronicles, let us for a moment consider the subsoil beneath cities that have been inhabited for many centuries. In London, for example, when excavations are made for drainage, building, or other purposes, there are sometimes found, many feet below the level of the present streets, mosaic pavements and foundations, together with earthen vessels, bronze implements, ornaments, coins, and other relics of Roman time. Now, if we knew nothing, from actual authentic history, of the existence of such a people as the Romans, or of their former presence in England, these discoveries, deep beneath the surface of modern London, would prove that long before the present streets were built, the site of the city was occupied by a civilised race which employed bronze and iron for the useful pur- poses of life, had a metal coinage, and showed not a little artistic skill in its pottery, glass, and sculpture. But down beneath the rubbish wherein the Roman remains are embedded, lie gravels and sands from which rudely-fashioned human implements of flint have been obtained. Whence we further learn that, before the civilised metal-using people appeared, an earlier race had been there, which employed weapons and instruments of roughly chipped flint. That this was the order of appearance of the successive peoples that have inhabited the site of London is, of course, obvious. But let us ask ourselves why it is obvious. We observe that there are, broadly speaking, three layers or deposits from which the evidence is derived. The upper layer is that which contains the foundations and rubbish of modern London. Next comes that which encloses the relics of the Roman occupation. At the bottom lies the layer that preserves the scanty traces of the early flint-folk. The upper deposit is necessarily the newest, for it could not be laid down until after the accumulation of those below it, which must, of course, be progressively older, as they are traced deeper from the surface. By the mere fact that the layers lie one above another, we are furnished with a simple clue which enables us to determine their relative time of formation. We may know nothing whatever as to how old they are measured by years or centuries. But we can be absolutely certain of what is termed their " order of superposition," or chronological sequence ; in other words, we can be confident that the bottom layer came first and the top layer last. 4 INTRODUCTORY CHAP. This kind of observation and reasoning will enable us to detect almost everywhere proofs that the surface of the land has not always been what it is to-day. In some districts, for example, when the dark layer of vegetable soil is turned up which supports the plants that keep the land so green, there may be found below it sand and gravel, full of smooth well-rounded stones. Such materials are to be seen in the course of formation where water keeps them moving to and fro, as on the beds of rivers, the margins of lakes, or the shores of the sea. Wherever smoothed rolled pebbles occur, they point to the influence of moving water ; so that we conclude, even though the site is now dry land, that the sand and gravel underneath it prove it to have been formerly under water. Again, below the soil in other regions, lie layers of oysters and other sea- shells. These remains, spread out like similar shells on the beach or bed of the sea at the present day, enable us to infer that where they lie the sea once rolled. Pits, quarries, or other excavations that lay open still deeper layers of material, bring before us interesting and impressive testimony regarding the ancient mutations of the land. Suppose, by way of further illustration, that underneath a bed of sand full of oyster-shells, there lies a dark brown band of peat. This substance, composed of mosses and other water-loving plants, is formed in boggy places by the growth of marshy vegetation. Below the peat there might occur a layer of soft white marl full of lake-shells, such as may be observed on the bottoms of many lakes at the present time (compare Fig. 30). These three layers oyster- bed, peat, and marl would present a perfectly clear and intelli- gible record of a curious series of changes in the site of the locality. The bottom layer of white marl with its peculiar shells would show that at one time the place was occupied by a lake. The next layer of peat would indicate that, by the growth of marshy vegetation, the lake was gradually changed into a morass. The upper layer of oyster-shells would prove that the ground was then submerged beneath the sea. The present condition of the ground shows that subsequently the sea retired and the locality passed into dry land as it is to-day. It is evident that by this method of examination information may be gathered regarding early conditions of the earth's surface, long before the authentic dates of human history. Such inquiries form the subject of Geology, which is the science that investigates the History of the Earth. The records in which this history is chronicled are the soils and rocks under our feet. It is the task I GEOLOGICAL METHODS 5 of the geologist so to arrange and interpret these records as to show through what successive changes the globe has passed, and how the dry land has come to wear the aspect which it presents at the present time. Just as the historian would be wholly unable to decipher the inscriptions of an ancient race of people unless he had first dis- covered a key to the language in which they are written, so the geologist would find himself baffled in his efforts to trace backward the history of the earth if he were not provided with a clue to the interpretation of the records in which that history is contained. Such a clue is furnished to him by a study of the operations of nature now in progress upon the earth's surface. Only in so far as he makes himself acquainted with these modern changes, can he hope to follow intelligently and successfully the story of earlier phases in the earth's progress. It will be seen that this truth has already been illustrated in the instances above given of the evidence that the surface of the land has not been always as it is now. The beds of sand and gravel, of oyster-shells, of peat and of marl, would have told us nothing as to ancient geography had we not been able to ascertain their origin and history by finding corresponding materials now in course of accumulation. To one ignorant of the peculiarities of fresh-water shells, the layer of marl would have conveyed no intelligible meaning. But knowing and recognising these peculiarities, we feel sure that the marl marks the site of a former lake. Thus the study of the Present supplies a key that unlocks the secrets of the Past. In order, therefore, to trace back the history of the Earth, the geologist must begin by carefully watching the changes that now take place, and by observing how nature elaborates the materials that preserve more or less completely the record of these changes. In the following pages, I propose to follow this method of inquiry, and, as far as the subject will permit, to start with no assumptions which the learner cannot easily verify for himself. We shall begin with the familiar everyday operations of the air, rain, frost, and other natural agents. As these have been fully described in my Class-Book of Physical Geography, it will not be needful here to consider them again in detail. We shall rather pass on to inquire in what various ways they are engaged in contributing to the forma- tion of new mineral accumulations, and in thereby providing fresh materials for the preservation of the facts on which geological history is founded. Having thus traced how new rocks are formed, we may then proceed to arrange the similar rocks of older 6 INTRODUCTORY CHAP. time, marking what are the peculiarities of each and how they may best be classified. If the labours of the geologist were concerned merely with the former mutations of the earth's surface, how sea and land have changed places, how rivers have altered their courses, how lakes have been filled up, how valleys have been excavated, how mountains, peaks, and precipices have been carved, how plains have been spread out, and how the story of these revolutions has been written in enduring characters upon the very framework of the land, he would feel the want of one of the great sources of interest in the study of the present face of nature. We naturally connect all modern changes of the earth's surface with the life of the plants and animals that flourish there, and more especially with their influence on the progress of Man himself. If there were no similar connection of the ancient changes with once living things if the history of the earth were merely one of dead inert matter it would lose much of its interest for us. But happily that history includes the records of successive generations of plants and animals which, from early times, have peopled land and sea. The remains of these organisms have been preserved in the deposits of different ages, and can be compared and con- trasted with those of the modern world. To realise how such preservation has been possible, and how far the forms so retained afford an adequate picture of the life of the time to which they belonged, we must turn once more to watch how nature deals with this matter at the present time. Of the millions of flowers, shrubs, and trees which year after year clothe the land with beauty, how many relics are preserved ? Where are the successive generations of insect, bird, and beast which have appeared in this country since man first set foot upon its soil ? They have utterly vanished. If all their living descendants could suddenly be swept away, how could we tell that such plants and animals ever lived at all ? It must be confessed that the vast majority of them leave no trace behind. Nevertheless we should be able to recover relics of some of them by searching in the comparatively few places where, at the present day, dead plants and animals are entombed and preserved. From the alluvial terraces of rivers, from the silt of lake-bottoms, from the depths of peat-mosses, from the floors of subterranean caverns, from the incrustations left by springs, we might recover traces of some at least of the living things that people the land. And from these fragmentary and incomplete records we might conjecture what i GEOLOGICAL RECORDS 7 may have been the general character of the life of the time. By searching the similar records of earlier ages the geologist has brought to light many profoundly interesting vestiges of vegetation and of animal life belonging to types that have long since passed away. It must be evident, however, that were we to confine our inquiries merely to its surface, we should necessarily gain a most imperfect view of the general history of the Earth. Beneath that surface, as volcanoes show, there lies a hot interior, which must have pro- foundly influenced the changes of the outer parts or crust of the planet. The study of volcanoes enables us to penetrate, as it were, a little way into that interior, and to understand some of the processes in progress there. But our knowledge of the inside of the Earth can obviously be based only to a very limited extent on direct observation, for man cannot penetrate far below the surface. The deepest mines do not go deep enough to reach materials differing in any essential respect from those visible above ground. Nevertheless, by inference from such observations as can be made, and by repeated and varied experiments in labora- tories, imitating as closely as can be devised what may be sup- posed to be the conditions that exist deep within the globe, some probable conclusions can be drawn even as to the changes that take place in those deeper recesses that lie for ever concealed from our eyes. These conclusions will be stated in later chapters of this book, and the rocks will be described, on the origin of which they appear to throw light. I have compared the soils and rocks with which geology deals to the records out of which the historian writes the chronicles of a nation. We might vary the simile by likening them to the materials employed in the construction of a great building. It is of course interesting enough to know what kinds of marble, granite, mortar, wood, brass, or iron, have been chosen by an architect. But much more important is it to inquire how these various substances have been grouped together so as to form such a building. In like manner, besides the nature and mode of origin of the various rocks of which the visible and accessible part of the earth consists, we ought to know how these varied substances have been arranged so as to build up what we can see of the outer part or crust of our globe. In short, we should try to trace what may be called the architecture of the planet, noting how each variety of rock occupies its own characteristic place, and how they are all grouped and braced together in the solid framework 8 INTRODUCTORY CHAP. of the land. This then will be the next subject for consideration in this volume. But in a great historical edifice, like one of the Gothic minsters of Europe, for example, there are often several different styles. A student of architecture can detect these distinctions, and by their means can show that a cathedral has not been completed in one age ; that it may even have been partially destroyed and re- built during successive centuries, only finally taking its present form after many political vicissitudes and many changes of architectural taste. Each edifice has thus a separate history, which is recorded by the way the materials have been shaped and put together in the various parts of the masonry. So it is with the architecture of the Earth. We have evidence of many demolitions and rebuild- ings, and the story of their general progress can still be deciphered among the rocks. It is the business of Geology to trace out that story, to put all the scattered materials together, and to make known by what a long succession of changes the Earth has reached its present state. An outline of what science has accom- plished in this task will form the last and concluding part of this book. In the following chapters I wish two principles to be kept steadily in view. In the first place, looking upon Geology as the study of the Earth's history, we need not at first concern ourselves with any details, save those that may be needed to enable us clearly to understand what the general character and progress of this history have been. In a science which embraces so vast a range as Geology, the multiplicity of facts to be examined and remembered may seem at first to be almost overwhelming. But a selection of the essential facts is sufficient to give the learner a clear view of the general principles and conclusions of the science, and to enable him to enter with intelligence and interest into more detailed treatises. In the second place, Geology is essentially a science of observation. The facts with which it deals should, as far as possible, be verified by our own personal examination. We should lose no opportunity of seeing with our own eyes the actual progress of the changes which it investigates, and the proofs which it adduces of similar changes in the far past. To do this will lead us into the fields and hills, to the banks of rivers and lakes, and to the shores of the sea. We can hardly take any country walk, indeed, in which with duly observant eye we may not detect either some geological operation in actual progress, or the evidence of one which was completed long ago. Having I INTEREST OF GEOLOGY 9 learnt what to look for and how to interpret it when seen, we are as it were gifted with a new sense. Every landscape comes to possess a fresh interest and charm, for we carry about with us everywhere an added power of enjoyment, whether the scenery has long been familiar or presents itself for the first time. I would therefore seek at the outset to impress upon those who propose to read the following pages, that one of the main objects with which this book is written is to foster a habit of observation, and to serve as a guide to what they are themselves to look for, rather than merely to relate what has been seen and determined by others. If they will so learn these lessons, I feel sure that they will never regret the time and labour they may spend over the task. PART I THE MATERIALS FOR THE HISTORY OF THE EARTH CHAPTER II THE INFLUENCE OF THE ATMOSPHERE IN THE CHANGES OF THE EARTH'S SURFACE IN the history of mankind no sharp line can be drawn between the events that are happening now or have happened within the last few generations, and those that took place long ago, and which are sometimes, though inaccurately, spoken of as historical. Every people is enacting its history to-day just as fully as it did many centuries ago. The historian recognises this continuity in human progress. He knows that the feelings and aspirations which guided mankind in old times were essentially the same influences that impel them now, and therefore that the wider his knowledge of his fellowmen of the present day, the broader will be his grasp in dealing with the transactions of former generations. So too is it with the history of the Earth. That history is in progress now as really as it has ever been, and its events are being recorded in the same way and by the same agents as in the far past. Its con- tinuity has never been broken. Obviously, therefore, if we would explore its records " in the dark backward and abysm of time," we should first make ourselves familiar with the manner in which these records are being written from day to day before our eyes. In this first Part, attention will accordingly be given to the changes in progress upon the Earth at the present time, and to the various ways in which the passing of these changes is chronicled CHAP, ii WEATHERING 11 in natural records. We shall watch the actual transaction of geological history, and ^mark in what way its incidents inscribe themselves on the page of the earth's surface. 1 Every day and hour witness the enacting of some geological event, trifling and transient or stupendous and durable. Sometimes the event leaves behind it only an imperceptible trace of its passage, at other times it graves itself almost imperishably in the annals of the globe. In tracing the origin and development of these geological annals of the present time, we shall best qualify ourselves for deciphering the records of the early revolutions of the planet. We are thereby led to study the various chronicles compiled respectively by the air, rain, rivers, springs, glaciers, the sea, plants and animals, volcanoes and earthquakes in other words, all the deposits left by the operations of these agents, the scars or other features made by them upon the earth's surface, and all other memorials of geological change. Having learnt how modern deposits are pro- duced, and how they preserve the story of their origin, we' shall then be able to group with them the corresponding deposits of earlier times, and to embrace all the geological records, ancient as well as modern, in one general scheme of classification. Such a scheme will enable us to see the continuity of the materials of geological history, and will fix definitely for us the character and relative position of all the chief rocks out of which the visible part of the globe is composed. Weathering. The gradual change that overtakes everything on the face of the earth is expressed in all languages by familiar phrases which imply that the mere passing of time is the cause of the change. As Sir Thomas Browne quaintly said more than two hundred years ago, " time antiquates antiquities, and hath an art to make dust of all things." We speak of the dust of antiquity and the gnawing Looth of time. We say that things are time- eaten, worn with age, crumbling under a weight of years. Nothing suggests such epithets so strikingly as an old building. We know that the masonry at first was smooth and fresh ; but now we describe it as weather-beaten, decayed, corroded. So distinctive is this appearance that it is always looked for in an ancient piece of stone-work ; and if not seen, its absence at once suggests a doubt whether the masonry can really be old. No matter of what 1 For descriptions of the ordinary operations of geological agents the reader is referred to my Class-Book of Physical Geography. My object now is to direct attention to what is most enduring in these operations, and in what various ways they form permanent geological records. 12 GEOLOGICAL WORK OF THE AIR CHAP. varieties of stone the edifice may have been built, a few generations may be enough to give them this look of venerable antiquity. The surface that was left smoothly polished by the builders grows rough and uneven, with scars and holes eaten into it. Portions of the original polish that may here and there have escaped, serve as a measure of how much has actually been removed from the rest of the surface. Now, if in the lapse of time, stone which has been artificially dressed is wasted away, we may be quite certain that the same stone in its natural position on the slope of a hill or valley, or by the edge of a river or of the sea, must decay in a similar way. Indeed, an examination of any crumbling building will show that, in proportion as the chiselled surface disappears, the stone puts on the ordinary look which it wears where it has never been cut by man, and where only the finger of time has touched it. Could we remove some of the FIG. i.-Weathering of rock, as shown by decayed stones f rom the building old masonry. (The "false-bedding" and other original structures of the stone are revealed by weathering.) and insert them into a natural crag or cliff of the same kind of stone, their peculiar time-worn aspect would be found to be so exactly that of the rest of the cliff that probably no one would ever suspect that a mason's tools had once been upon them. From this identity of surface between the time-worn stones of an old building and the stone of a cliff we may confidently infer that the decay so characteristic of ancient masonry is as marked upon patural faces of rock. The gradual disappearance of the artificial smoothness given by the mason, and its replacement by the ordinary natural rough surface of the stone, shows that this natural surface must also be the result of decay. And as the peculiar crumbling character is universal, we may be sure that the decay with which it is connected must be general over the globe. But the mere passing of time obviously cannot change any- thing, and to say that it does is only a convenient figure of speech. It is not time, but the natural processes which require time for ii CAUSES OF WEATHERING 13 their work, that produce the widespread decay over the surface of the earth. Of these natural processes, there are four that specially deserve consideration changes of temperature, saturation and desiccation, frost, and rain. (1) Changes of Temperature. In countries where the days are excessively hot, with nights correspondingly cool, the surfaces of rocks heated sometimes, as in parts of Africa, up to more than 130 Fahr. by a tropical sun, undergo considerable expansion in consequence of this increase of temperature. At night, on the other hand, the rapid radiation quickly chills the stone and causes it to contract. Hence the superficial parts, being in a perpetual state of strain, gradually crack up or peel off. The face of a cliff is thus worn slowly backward, and the prostrate blocks that fall from it are reduced to smaller fragments and finally to dust. Where, as in Europe and the settled parts of North America, the contrasts of temperature are not so marked, the same kind of waste takes place in a less striking manner. (2) Saturation and Desiccation. Another cause of the decay of the exposed surfaces of rocks is to be sought in the alter- nate soaking of them with rain and drying of them in sunshine, whereby the component particles of the stone are loosened and fall to powder. Some kinds of stone freshly quarried and left to this kind of action are rapidly disintegrated. The rock called shale (see p. 153) is peculiarly liable to decay from this cause. The cliffs into which it sometimes rises show at their base long trails of rubbish entirely derived from its waste. (3) Frost. A third and familiar source of decay in stone exposed to the atmosphere is to be found in the action of Frost The water that falls from the air upon the surface of the land soaks into the soil and into the pores of rocks. When the temperature of the air falls below the freezing point, the imprisoned moisture expands as it passes into ice, and in expanding pushes aside the particles between which it is entangled. Where this takes place in soil, the pebbles and the grains of sand and earth are separated from each other by the ice that shoots between them. They are all frozen into a solid mass that rings like stone under our feet ; but, as soon as a thaw sets in, the ice that formed the binding cement passes into water which converts the soil into soft earth or mud. This process, repeated winter after winter, breaks up the materials of the soil, and enables them to be more easily made use of by plants and more readily blown away by wind or washed off by rain. Where the action of frost affects the surface i 4 GEOLOGICAL WORK OF THE AIR CHAP. of a rock, the particles separated from each other are eventually blown or washed away, or the rock peels off in thin crusts or breaks up into angular pieces, which are gradually disintegrated and removed. (4) Rain. One further cause of decay may be sought in the re- markable power possessed by Rain of chemically corroding stones. In falling through the atmosphere, rain absorbs the gases of the air, and with their aid attacks surfaces of rock. With the oxygen thus acquired, it oxidises those substances which can still take more of this gas, causing them to rust (pp. 117, 123). As a consequence of this alteration, the cohesion of the particles is usually weakened, and the stone crumbles down. With the carbon-dioxide, or car- bonic acid, it dissolves and removes some of the more soluble ingredients in the form of carbonates, thereby also usually loosen- ing the component particles of the stone. In general, the influence of rain is to cause the exposed parts of rocks to rot from the sur- face inward. Where the ground is protected with vegetation, the decay is no doubt retarded ; but in the absence of v.egetation, the outer crust of the decayed layer is apt to be washed off by rain, or when dried to powder may be blown away and scattered by wind. As fast as it is removed from the surface, however, it is renewed underneath by the continued soaking of rain into the stone. Effects of Weathering. Hence one of the first lessons to be learnt when from the common evidence around us we seek to know what has been the history of the ground on which we live- is one of ceaseless decay. All over the land, in all kinds of climates, and from various causes, bare surfaces of soil and rock yield to the influences of the atmosphere or weather. The decay thus set in motion is commonly called " weathering." That it may often be comparatively rapid is familiarly and instructively shown in buildings or open-air monuments of which the dates are precisely known. Marble tombstones in the graveyards of large towns, for example, hardly keep their inscriptions legible for even so long as a century. Before that time, the surface of the stone has crumbled away into a kind of sand. Everywhere the weather- eaten surfaces, the crumbling crust of decayed stone, and the scattered blocks and trains of rubbish, tell their tale of universal waste. It is well to take numerous opportunities of observing the pro- cess of this decay in different situations and on various kinds of materials. We can thus best realise the important part which weathering must play in the changes of the earth's ^urface, and ii EFFECTS OF WEATHERING 15 we prepare ourselves for the consideration of the next question that arises, What becomes of all the rotted material ? a question to answer which leads us into the very foundations of geological history. Openings from the soil down into the rock underneath often afford instructive lessons regarding the decay of the surface of the land. Fig. 2, for instance, is a drawing of one of these sections, in which a gradual passage may be traced from solid sandstone (a) under- neath up into broken -up sandstone (), and thence into the earthy layer (c) that supports the vegetation of the surface. Traced from below upwards, the rock is found to become more and more broken and crumbling, with an increasing number of rootlets that strike freely through it in all directions, until it passes insensibly into the uppermost dark layer of vege- table soil Or humus. This dark layer FIG. a .-Passage of sandstone . . * upwards into soil. owes its characteristic brown or black colour to the decaying remains of vegetation diffused through it. Again, granite in its unweathered state is a hard, compact, crystal- line rock that may be quarried out in large solid blocks (a in Fig. 3), yet when traced upward to within a few feet from the surface it may be seen to have been split by innumerable rents into fragments - which are never- theless still lying in their original position. As these fragments are attacked by percolating moisture, their surfaces decay, leaving the still unweathered parts as rounded blocks (b\ which might at first be mistaken for transported boulders. They are, however, parts of the rock broken up in place, and not fragments that have been carried from a distance. ,The little quartz veins that traverse the solid granite can be recognised running through the FIG. 3. Passage of granite upwards into soil. 16 GEOLOGICAL WORK OF THE AIR CHAP. decayed and fresh parts alike. But, besides being broken into pieces, the granite rots away and loses its cohesion. Some of the smaller pieces can be crumbled down between the fingers, and this decay increases upwards, until the rock becomes a mere sand or sandy clay in which a few harder kernels are still left. Into this soft layer roots may descend from the surface, and, like the sandstone, the granite merges above into the overlying soil (c). Soil and Subsoil. In such sections as the foregoing, three distinct layers can be recognised which pass into each other. At the bottom lies the rock^ either undecayed or at least still fresh enough to show its true nature. Next comes the broken -up crumbling layer through which stray roots descend, and which is known as the subsoil. At the top lies the dark band, crowded with rootlets and forming the true soil. These three layers obviously represent successive stages in the decay of the surface of the land. The soil is the layer of most complete decay. The subsoil is an intermediate band where the progress of decomposi- tion has not advanced so far, while the shattered rock underneath shows the earlier stages of disintegration. Vegetation sends its roots and rootlets through the rotted rock. As the plants die, they are succeeded by others, and the rotted remains of their successive generations gradually darken the uppermost decom- posed layer. Worms, insects, and larger animals that may die on the surface, likewise add their mouldering remains to this uppermost deposit. And thus from animals and plants there is furnished to the soil that organic matter on which its fertility so much depends. The very decay of the vegetation helps to promote that of the underlying rock, for it supplies various organic acids ready to be absorbed by percolating rain-water, the power of which to decompose rocks is thereby increased (p. 24). It is obvious, then, that in answer to the question, What becomes of the rotted material produced by weathering ? we may confidently assert that, over surfaces of land protected by a cover of vegetation, this material in large measure accumulates where it is formed. Such accumulation will naturally take place chiefly on flat or gently inclined ground. W T here the slope is steep, the decomposed layer will tend to travel down-hill by mere gravitation, and to be further impelled downward by descending rain-water. If there is so intimate a connection between the soil at the surface and the rock underneath, we can readily understand that soils should vary from one district to another, according to the nature of the underlying rocks. Clays will produce clayey soil, ii SOIL SUBSOIL RAIN 17 sandstones, sandy soil, or, where these two kinds of rock occur together, they may give rise to sandy clay or loam. Hence, knowing what the underlying rock is, we may usually infer what must be the character of the overlying soil, or, from the nature of the soil, we may form an opinion respecting the quality of the rock that lies below. But it will probably occur to the thoughtful observer that when once a covering of soil and subsoil has been formed over a level piece of ground, especially where there is also an overlying carpet of verdure, the process of decay should cease the very layer of rotted material coming eventually to protect the rock from further disintegration. Undoubtedly, under these circumstances, weather- ing is reduced to its feeblest condition. But that it still continues will be evident from some considerations, the force of which will be better understood a few pages further on. If the process were wholly arrested, then in course of time plants growing on the surface would extract from the soil all the nutriment they could get out of it, and with the increasing impoverishment of the soil, they would dwindle away and finally die out, until perhaps only the simpler forms of vegetation would grow on the site. Some- thing of this kind not improbably takes place where forests decay and are replaced by scrub and grass. But the long-continued vigorous growth of the same kind of plants upon a tract of land doubtless indicates that in some way the process of weathering is nob entirely arrested, but that, as generation succeeds generation, the plants are still able to draw nutriment from fresh portions of decomposed rock. A cutting made through the soil and subsoil shows that roots force their way downward into the rock, which splits up and allows percolating water to soak downwards through it. The subsoil thus gradually eats its way into the solid rock below. Influences are at work also, whereby there is an imper- ceptible removal of material from the surface of the soil. Notable among these influences are Rain, Wind, and Earthworms. Wherever soil is bare of vegetation it is directly exposed to removal by Rain. Ground is seldom so flat that rain may not flow a little way along the surface before sinking underneath. In its flow, it carries off the finer particles of the soil. These may travel each time only a short way, but as the operation is repeated, they are in the course of years gradually moved down to lower ground or to some runnel or brook that sweeps them away seaward. Both on gentle and on steep slopes, this transporting power of rain is continually removing the upper layer of bared soil. C i8 GEOLOGICAL WORK OF THE AIR CHAP. Where soil is exposed to the sun, it is liable to be dried into mere dust, which is borne off by Wind. How readily this may happen is often strikingly seen after dry weather in spring-time. The earth of ploughed fields becomes loose and powdery, and clouds of its finer particles are carried up into the air and trans- ported to other farms, as gusts of wind sweep across. " March dust," which is a proverbial expression, may be remembered as an illustration of one way in which the upper parts of the soil are , removed. Even where a grassy turf protects the general surface, bare places may always be found whence this covering has been removed. Rabbits, moles, and other animals throw out soil from their burrows. Mice sometimes lay it bare by eating the pasture down to the roots. The common Earthworms bring up to day- light in the course of a year an almost incredible quantity of it in their castings. Mr. Darwin estimated that this quantity is in some places not less than 10 tons per annum over an acre of ground. Only the finest particles of mould are swallowed by worms and conveyed by them to the surface, and it is precisely these which are most apt to be washed off by rain or to be dried and blown away as dust by the wind. Where it remains on the ground, the soil brought up by worms covers over stones and other objects lying there, which consequently seem to sink into the earth. The operation of these animals causes the materials of the soil to be thoroughly mixed. In tropical countries, the termite or " white ant " conveys a prodigious amount of fine earth up into the open air. With this material it builds hills sometimes 60 feet high and visible for a distance of several miles ; likewise tunnels and chambers, which it plasters all over the stems and branches of trees, often so continuously that hardly any bark can be seen. The fine soil thus exposed is liable to be blown away by the wind or washed off by the fierce tropical rains. Although, therefore, the layer of vegetable soil which covers the land appears to be a permanent protection, it does not really prevent a large amount of material from being removed even from grassy ground. It forms the record of the slow and almost imperceptible geological changes that affect the regions where it accumulates, the quiet fall of rain, the gradual rotting away of the upper part of the underlying rock, the growth and decay of a long succession of generations of plants, the ceaseless labours of the earthworm, the scarcely appreciable removal of material from the surface by the action of rain and wind, and the equally ii TALUS -SLOPES, RAIN -WASH 19 insensible descent of the crumbling subsoil farther and farther into the solid stone below. Having learnt how all this is told by the soil beneath our feet, we should be ready to recognise in the soil of former ages a similar chronicle of quiet atmospheric dis- integration. Talus. Besides soil and subsoil, there are other forms in which decomposed rock accumulates on the surface of the land. Where a large mass of bare rock rises up as a steep bank or cliff, it is liable to constant degradation, and the materials detached from its surface accumulate down the slopes, forming what is known as a Talus (Fig. 4). In mountainous or hilly regions, FIG. 4. Talus-slopes at the foot of a line of cliffs. where rocky precipices rise high into the air, there gather at their feet and down their clefts long trails or screes of loose blocks that have been split off from them by the weather. Such slopes, especially where they are not too steep, and where the rubbish that forms them is not too coarse, may be more or less covered with vegetation, which in some measure arrests the descent of the debris. But from time to time, during heavy rains, deep gullies are torn out of them by rapidly formed torrents, which sweep down their materials to lower levels (Fig. 10). The sections laid bare in these gullies show that the rubbish is arranged in more or less distinct layers which lie generally parallel with the surface of the slope ; in other words, it is rudely stratified, and its layers or strata are inclined at the angle of the declivity which seldom exceeds 35. Rain-wash, Brick-earth. On more gentle slopes, even where no bare rock projects into the air, the fall of rain gradually washes down the upper parts of the soil to lower levels. Hence 20 GEOLOGICAL WORK OF THE AIR CHAP. arise thick accumulations of what is known as rain-wash soil mixed often with angular fragments of still undecomposed rock, and not infrequently forming a kind of brick -earth (Fig. 5). Deposits of this nature are still gathering now, though their lower portions may be of great antiquity. In the south-east of England, for instance, the brick -earths contain the bones of animals that have long since passed away. Dust. By the action of wind, above referred to, a vast amount of fine dust and sand is carried up into the air and strewn . , , far and wide over the land. In dry FIG. 5 -Section of rain-wash coimtr}e s, such as large tracts of Central or brick-earth. 7. Vegetable . , . . -,... soil. 6. Brick-earth. 5. White Asia, the air is often thick with a fine sand. 4. Brick - earth. 3 . yellow dust which may entirely obscure the White sand. 2 . Brick-earth. sun at m id-day, and which settles over posit, which may be hundreds of feet deep, is thus accumulated on the surface of the land. Some of the ancient cities of the Old World, Nineveh and Babylon for example, after being long abandoned by man, have gradually been buried under the fine soil drifted over them by the wind and intercepted and protected by the weeds that grew up over the ruins. Even in regions where, as in Britain, there is a large annual rainfall, seasons of drought occur, during which there may be a considerable drifting of the finer particles of soil by the wind. We probably hardly realise how much the soil may be removed here and heightened there from this cause. Sand-dunes. Some of the most strikingand familiar examples of the accumulation of loose deposits by the wind are those to which the name of Dunes is given. On sandy shores, exposed to winds that blow landwards, the sand is dried and then carried away from the beach, gathering into long mounds or ridges which run parallel to the coast-line. These ridges are often 50 or 60 feet^. sometimes even more than 250 feet high, with deep troughs and irregular circular hollows between them, and they occasionally form a strip several miles broad, bordering the sea. The particles of sand are driven inland by the wind, and the dunes gradually bury fields, roads, and villages, unless their progress is arrested by the growth of vegetation over their shifting surfaces. On many parts of the west coast of Europe, the dunes are marching II SAND-DUNES 21 into the interior at the rate of 20 feet in a year. Hence large tracts of land have within historic times been entirely lost under them. In the north of Scotland, for example, an ancient and extensive barony, so noted for its fertility that it was called " the granary of Moray," was devastated about the middle of the seven- teenth century by the moving sands, which now rise in barren ridges more than 100 feet above the site of the buried land. In the interior of continents also, where with great dryness of climate there is a continual disintegration of the surface of rocks, wide wastes of sand accumulate, as in the deserts of Libya, Arabia, FIG. 6. Sand-dunes. and Gobi, in the heart of Australia, and in many of the western parts of the United States. There can be no doubt, however, that though the layer of vegetable soil, the heaps of rubbish that gather on slopes and at the base of rocky banks and precipices, and the widespread drifting of dust and sand over the land, afford evidence that much of the material arising from the general decay of the surface of the land accumulates under various forms upon that surface, nevertheless its stay there is not permanent. Wind and rain are continually removing it, sometimes in vast quantities, into the sea. Every brook, made muddy by heavy rain, is an example of this transport, for the mud that discolours the water is simply the finer material of the soil washed off by rain. When we reflect upon 22 GEOLOGICAL WORK OF THE AIR CHAP, n the multitude of streams, large and small, in all parts of the globe, and consider that they are all busy carrying their freights of mud to the sea, we can in some measure appreciate how great" must be the total annual amount of material so removed. What becomes of this material will form the subject of succeeding chapters. Summary. The first lesson to be learnt from an examination of the surface of the land is, that everywhere decay is in progress upon it. Wherever the solid rock rises into the air, it breaks up and crumbles away under the various influences combined in the process of Weathering. The wasted materials caused by this universal disintegration partly accumulate where they are formed, and make soil. But in large measure, also, they are blown away by wind and washed off by rain. Even where they appear to be securely protected by a covering of vegetation, the common earth- worm brings the finer parts of them up to the surface, where they come within reach of rain and wind, so that on tracts permanently grassed over, there may be a continuous and not inconsiderable removal of fine soil from the surface. In proportion as the upper layers of soil are removed, roots and percolating water are enabled to reach down farther into the solid rock which is broken up into subsoil, and thus the general surface of the land is insensibly lowered. Besides accumulating in situ as subsoil and soil, the debris of decomposed rock forms talus -slppes and screes at the foot of crags, and a layer of rain-wash or brick-earth over gentler slopes. Where the action of wind comes markedly into play, tracts of sand-dunes may be piled up along the borders of the sea and of lakes, or in the arid interior of continents ; and wide regions have been in course of time buried under the fine dust which is some- times so thick in the air as to obscure the noonday sun. But in none of these forms can the accumulation of decomposed material be regarded as permanent. So long as it is exposed to the influences of the atmosphere, this material is still liable to be swept away from the surface of the land and borne outwards into the sea. CHAPTER III THE INFLUENCE OF RUNNING WATER IN GEOLOGICAL CHANGES, AND HOW IT IS RECORDED IT appears, then, that from various causes all over the globe, there is a continual decay of the surface of the land ; that the decom- posed material partly accumulates as soil, subsoil, and sheets or heaps of loose earth or sand, but that much of it is washed off the land by rain or blown into the rivers or into the sea by wind. We have now' to consider the part taken by Running Water in this transport. From the single rain-drop up to the mighty river, every portion of the water that flows over the land is busy with its own share of the work. When we reflect on the amount of rain that falls annually over the land, and on the number of streams, large and small, that are ceaselessly at work, we realise how difficult it must be to form any fit notion of the entire amount of change which, even in a single year, these agents work upon the surface of the earth. The influence of rain in the decay of the surface of the land was briefly alluded to in the last chapter. As soon as a drop of rain reaches the ground, it begins its appointed geological task, dissolving what it can carry off in solution, and pushing forward and downward whatever it has power to move. As the rain-drops gather into runnels, the same duty, but on a larger scale, is performed by them ; and as the runnels unite into large streams, and these into yet mightier rivers, the operations, though becoming colossal in magnitude, remain essentially the same in kind. In the operations of the nearest brook, we see before us in miniature a sample of the changes produced by the thousands of rivers which, in all quarters of the globe, are flowing from the mountains to the sea. Watching these operations from day to day, we discover that they may all be classed under two heads. In the first place, 24 .RECORDS OF RUNNING WATER CHAP. the brook hollows out the channel in which it flows and thus aids in the general waste of the surface of the land ; and in the second place, it carries away fine silt and other material resulting from that waste, and either deposits it again on the land or carries it out to sea. Rivers are thus at once agents that themselves directly degrade the land, and that sweep the loosened detritus - towards the ocean. An acquaintance with each of these kinds of work is needful to enable us to understand the nature of the records which river-action leaves behind it. i. EROSIVE AND TRANSPORTING POWER OB' RUNNING WATER. Chemical Action. We have seen that rain in its descent from the clouds absorbs air, and that with the oxygen and carbonic acid which it thus obtains it proceeds to corrode the surfaces of rock on which it falls. When it reaches the ground and absorbs the acids termed " humous," which are supplied by the decomposing vegetation of the soil, it acquires increased power of eating into the stones over which it flows. When it rolls along as a runnel, brook, or river, it no doubt still attacks the rocks of its channel, though its action in this respect is not so easily detected. In some circumstances, however, the solvent influence of river-water upon solid rocks is strikingly displayed. Where the water contains a large proportion of the acids of the soil, and flows over a kind of rock specially liable to be eaten away by these acids, the most favourable conditions are presented for observing the change. Thus, a stream which issues from a peat-bog is usually dark brown in colour, from the vegetable solutions which it extracts from the moss. Among these solutions are some of the organic acids referred to, ready to eat into the surface of the rocks or loose stones which the stream may encounter in its descent. No kind of rock is more liable than limestone to corrosion under such circumstances. Peaty water flowing over it eats it away with comparative rapidity, while those portions of the rock that rise above the stream escape solution, except in so far as they are attacked by rain. Hence arise some curious features in the scenery of limestone districts. The walls of limestone above the water, being attacked only by the atmosphere, are not eaten away so fast as their base, over which the stream flows. They are consequently undermined, and are sometimes cut into dark tunnels and passages (Fig. 7). Even where the solvent action of the water of rivers is otherwise inappreciable, it can be detected by Ill CHEMICAL ACTION 25 means of chemical analysis. Thus rivers, partly by the action of their water upon the loose stones and solid rocks of their channels, and partly by the contributions they receive from Springs (which will be afterwards described), convey a vast amount of dissolved material into the sea. The mineral substance thus invisibly transported consists of various salts. One of the most abundant of these carbonate of lime is the substance that forms lime- stone, and furnishes the mineral matter required for the hard parts of a large proportion of the lower animals. It is a matter of some interest to know that this substance, so indispensable for the FIG. 7. Erosion of limestone by the solvent action of a peaty stream, Durness, Sutherlandshire. formation of the shells of so large a number of sea -creatures, is constantly supplied to the sea by the streams that flow into it. 1 The rivers of Western Europe, for instance, have been ascertained to convey about I part of dissolved mineral matter in every 5000 parts of water, and of this mineral matter about a half consists of carbonate of lime. It has been estimated that the Rhine bears enough carbonate of lime into the sea every year to make three hundred and thirty-two thousand millions of oysters of the usual size. Another abundant ingredient of river-water is gypsum or 1 There is now reason, however, to suspect that the carbonate of lime in marine organisms is not derived so much from the comparatively minute proportion of that substance present in solution in sea-water, as from the much more abundant sulphate of lime which undergoes apparently a process of chemical transformation into carbonate within the living animals. 26 RECORDS OF RUNNING WATER CHAP. sulphate of lime, of which the Thames is computed to carry annually past London not less than 180,000 tons. The total quantity of carbonate of lime, removed from the limestones of its basin by this river in a year, amounts, on an average, to 140 tons from every square mile, which is estimated to be equal to the lowering of the general surface to the extent of T |^ of an inch from each square mile in a century, or one foot in 13,200 years. Mechanical Action (i) Transport. The dissolved material forms but a small proportion of the total amount of mineral sub- stances conveyed by rivers from land to sea. A single shower of rain washes off fine dust and soil from the surface of the ground into the nearest brook which then rolls along with a discoloured current. An increase in the volume of the water enables a stream to sweep along sand, gravel, and blocks of stone lying in its channel, and to keep these materials moving until, as the declivity lessens and the rain ceases, the current becomes too feeble to do more than lazily carry onward the fine silt that discolours it. Every stream, large or small, is ceaselessly busy transporting mud, sand, or gravel. And as the ultimate destination of all this sediment is the bottom of the sea, it is evident that if there be no compensating influences at work to repair the constant loss, the land must in the end be all worn away. Some of the most instructive lessons regarding the work of running water on land are afforded by the beds of mountain- torrents. Huge blocks, detached from the crags and cliffs on either side, may there be seen cumbering the pathway of the water, which seems quite powerless to move such masses and can only sweep round them or find a passage beneath them. But visit such a torrent when it is swollen with heavy rains or rapidly melted snow, and you will hear the stones knocking against each other or on the rocky bottom, as they are driven downwards by the flood. Or when the stream is at its lowest, in dry summer weather, follow its course a little way down hill, and you will see that by degrees the blocks, losing their sharp edges, have become rounded boulders, and that these are gradually replaced by coarse shingle and then by finer gravel. In the quieter reaches of the water, sheets of sand begin to make their appearance, and at last when the stream reaches the plains, no sediment of coarser grain than mere silt may be seen in its channel. It is thus obvious that in the constant transport maintained by watercourses, the carried materials, by being rolled along rocky channels and continually ground against each other, diminish in size as they descend. A in MECHANICAL ACTION 27 river flowing from a range of mountains to the distant ocean may be likened to a mill, into which large angular masses of rock are cast at the upper end, and out of which only fine sand and silt are discharged at the lower. Partly, therefore, owing to the fine dust and soil swept into them by wind and rain from the slowly decomposing surface of the land, and partly to the friction of the detritus which they sweep along their channels, rivers always contain more or less mineral matter suspended in their water or travelling with the current on the bottom. The amount of material thus transported varies greatly in different rivers, and at successive seasons even in the same river. In some cases, the rainfall is spread so equably through the year that the rivers flow onward with a quiet monotony, never rising much above nor sinking much below their average level. In such circumstances, the amount of sediment they carry downward is proportionately small. On the other hand, where either from heavy periodical rains or from rapid melting of snow, rivers are liable to floods, they acquire an enormously increased power of transport, and their burden of sediment is proportionately augmented. In a few days or weeks of high water, they may convey to the sea a hundredfold the amount of mineral matter which they could carry in a whole year of their quieter mood. Measurements have been made of the proportions of sediment in the waters of different rivers at various seasons of the year. The results, as might be expected," show great variations. Thus the Garonne, rising among the higher peaks of the Pyrenees, drains a large area of the south of France, and is subject to floods by which an enormous quantity of sediment is swept down from the mountains to the plains. Its proportion of mud has been estimated to be as much as I part in 100 parts of water. The Durance, which takes its source high on the western flank of the Cottian Alps, is one of the rapidest and muddiest rivers in Europe. Its angle of slope varies from I in 208 to i in 467, the average declivity of the great rivers of the globe being probably not more than i in 2600, while that of a navigable stream ought not to exceed 10 inches per mile or i in 6336. The Durance is, there- fore, rather a torrent than a river. With this rapidity of descent is conjoined an excessive capacity for transporting sediment. In floods of exceptional severity, the proportion of mud in the stream has been estimated at one-tenth by weight of the water, while the average proportion for nine years from 1867 to 1875 was about Probably the best general average is to be obtained from a 28 RECORDS OF RUNNING WATER CHAP. river which drains a wide region exhibiting considerable diversities of climate, topography, rocks, and soils. The Mississippi presents a good illustration of these diversities, and has accordingly been taken as a kind of typical river, furnishing, so to speak, a standard by which the operations of other rivers may be compared, and which may perhaps be assumed as a fair average for all the rivers of the globe. Numerous measurements have been made of the proportion of sediment carried into the Gulf of Mexico by this vast river, with the result of showing that the average amount of sediment is by weight I part in every i 500 parts of water, or little more than one-third of the proportion in the water of the Durance. If now we assume that, all over the world, the general average proportion of sediment floating in the water of rivers is I part in every 1500 of water, we can readily understand how seriously in the course of time must the land be lowered by the constant removal of so much decomposed rock from its surface. Knowing the area of the basin drained by a river, and also the proportion of sediment in its water, we can easily calculate the general loss from the surface of the basin. The ratio of the weight or " specific gravity " of the silt to that of solid rock may be taken to be as 19 is to 25. Accordingly the Mississippi conveys annually from its drainage basin an amount of sediment equivalent to the removal of -Q-^-Q-Q part of a foot of rock from the general surface of the basin. At this rate, one foot of rock will be worn away every 6000 years. If we take the general height of the land of the whole globe to be 21 20 feet, and suppose it to be continuously wasted at the same rate at which the Mississippi basin is now suffering, then the whole dry land would be carried into the sea in 12,720,000 years. Or if we assume the mean height of Europe to be 973 feet and that this continent is degraded at the Mississippi rate of waste until the last vestige of it disappears, the process of destruction would be completed in rather less than 6,000,000 years. Such estimates are not intended to be close approximations to the truth. As the land is lowered, the rate of decay will gradually diminish, so that the later stages of decay will be enormously protracted. But by taking the rate of opera- tion now ascertained to be in progress in such a river basin as the Mississippi, we obtain a valuable standard of comparison, and learn that the degradation of the land is much greater and more rapid than might have been supposed. (2) Erosion. But rivers are not merely carriers of the mud, sand, and gravel swept into their channels by other agencies. By in EROSION 29 keeping these materials in motion, the currents reduce them in size, and at the same time employ them to hollow out the channels wherein they move. The mutual friction that grinds down large blocks of rock into sand and mud, tells also upon the rocky beds along which the material is driven. The most solid rocks are worn down deep long gorges are dug out, and the water- courses, when they have once chosen their sites, remain on them and sink gradually deeper and deeper beneath the general level of the country. The surfaces of stone exposed to this attrition assume the familiar smoothed and rounded appearance which is known as water-worn. The loose stones lying in the channel of a stream, and the solid rocks as high up as floods can scour them, present this characteristic aspect. Here and there, where a few stones have been caught in an eddy of the current, and are kept in constant gyration, they reduce each other in dimensions, and at the same time grind out a hollow in the underlying rock. The sand and mud produced by the friction are swept off by the current, and the stones when sufficiently reduced in size are also carried away. But their places are eventually taken by other blocks brought down by floods, so that the supply of grinding material is kept up until the original hollow is enlarged into a wide deep caldron, at the bottom of which the stones can only be stirred by the heaviest floods. Cavities of this kind, known as pot-holes, are of frequent occurrence in rocky watercourses as well as on rocky shores, in short, wherever eddies of water can keep shingle rotating upon solid rock. As they often coalesce by the wearing away of the intervening wall of rock, they greatly aid in the deepening of a watercourse. In most rocky gorges, a succes- sion of old pot-holes may be traced far above the present level of the stream (Fig. 8). That it is by means of the gravel and other detritus pushed along the bottom by the current, rather than by the mere friction of the water on its bed, that a river excavates its channel, is most strikingly shown immediately below a lake. In traversing a lake, the tributary streams deposit their sediment on its bottom, because the still water checks their current and, by depriving the water of its more rapid movement, compels it to drop its burden of gravel, sand, and silt (see p. 42). Filtered in this way, the various streams united in the lake escape at its lower end as a clear trans- parent river. The Rhone, for instance, flows into the Lake of Geneva as a turbid stream ; it issues from that great reservoir at Geneva as a rushing current of the bluest, most translucent water 30 RECORDS OF RUNNING WATER CHAP. which, though it sweeps over ledges of rock, has not yet been able to grind them down into a deep gorge. The Niagara, also, filtered by Lake Erie, has not acquired sediment enough to enable it to cut deeply into the rocks over which it foams in its rapids before throwing itself over the great Falls. One of the most characteristic features of streams is the singularly sinuous courses they follow. As a rule, too, the flatter ; ;,.:,,., ,.,.... -..- ; - FIG. 8. Pot-holes worn out by the gyration of stones in the bed of a stream. the ground over which they flow, the more do they wind. Not uncommonly they form loops, the nearest bends of which in the end unite, and as the current passes along the now straightened channel, the old one is left to become by degrees a lake or pond of stagnant water, then a marsh, and lastly, dry ground. We might suppose that in flowing off the land, water would take the shortest and most direct road to the sea. But this is far from being the case. The slightest inequalities of level have originally determined sinuftsities of the channels, while trifling differences in the hardness of the banks, in the accumulation of sediment, and in EROSION OF RIVER -CHANNELS 31 in the direction of the currents and eddies, have been enough to turn a stream now to one side now to another, until it has assumed its present meandering course. How easily this may be done can be instructively observed on a roadway or other bare surface of ground. When quite dry and smooth, hardly any depressions in which water would flow might be detected on such a surface. But after a heavy shower of rain, runnels of muddy water will be seen coursing down the slope in serpentine channels that at once recall the winding rivers of a great drainage -system. The slightest differences of level have been enough to turn the water from side to side. A mere pebble or projecting heap of earth or tuft of grass has sufficed to cause a bend. The water, though always descending, has only been able to reach the bottom by keeping the lowest levels, and turning from right to left as these guided it. When a river has once taken its course and has begun to ex- cavate its channel, only some great disturbance, such as an earth- quake or volcanic eruption, can turn it out of that course. If its original pathway has been a winding one, it goes on digging out its bed which, with all its bends, gradually sinks below the level of the surrounding country. The deep and picturesque gorge in which the Moselle winds from Treves to Coblenz has in this way been slowly eroded out of the undulating tableland across which the river originally flowed. In another and most characteristic way, the shape of the ground and the nature and arrangement of the rocks over which they flow, materially influence rivers in the forms into which they carve their channels. The Rhone and the Niagara, for instance, though filtered by the lakes through which they flow, do not run far before plunging into deep ravines. Obviously such ravines cannot have been dug out by the same process of mechanical attrition whereby river-channels in general are eroded. Yet the frequency of gorges in river scenery shows that they cannot be due to any exceptional operation. They may generally be accounted for by some arrange- ment of rocks wherein a bed of harder material is underlain by one more easily removable. Where a stream, after flowing over the upper bed, encounters the decomposable bed below, it eats away the latter more rapidly. The overlying hard rock is thus under- mined, and, as its support is destroyed, slice after slice is cut away from it. The waterfall which this kind of structure produces con- tinues to eat its way backward or up the course of the stream, so long as the necessary conditions are maintained of hard rocks lying upon soft. Any change of structure which would bring the 32 RECORDS OF RUNNING WATER CHAP. hard rocks down to the bed of the channel, and remove the soft rocks from the action of the current and the dash of the spray, would gradually destroy the waterfall. It is obvious that, by cut- ting its way backward, a waterfall excavates a ravine. The renowned Falls of Niagara supply a striking illustration of the process now described. The vast body of water which issues from Lake Erie, after flowing through a level country for a few miles, rushes down its rapids and then plunges over a precipice of solid limestone. Beneath this hard rock lies a band of com- paratively easily eroded shale. As the water loosens and removes the lower rock, large portions of the face of the precipice behind the Falls are from time to time precipitated into the boiling flood below. The waterfall is thus slowly prolonging the ravine below the Falls. The magnificent gorge in which the Niagara, after its tumultuous descent, flows sullenly to Lake Ontario is not less than 7 miles long, from 200 to 400 yards wide, and from 200 to 300 feet deep. There is no reason to doubt that this chasm has been entirely dug out by the gradual recession of the Falls from the cliffs at Queenstown, over which the river at first poured. We may form some conception of the amount of rock thus removed from the estimate that it would make a rampart about 1 2 feet high and 6 feet thick extending right round the whole globe at the equator. Still more gigantic are the gorges or canons of the Colorado and its tributaries in Western America. The Grand Canon of the Colorado is 300 miles long, and in some places more than 6000 feet deep (Fig. 9). The country traversed by it is a network of profound ravines, at the bottom of which the streams flow that have eroded them out of the table-land. ii. DEPOSITION OF MATERIALS BY RUNNING WATER. Permanent Records of River-Action. If, then, all the streams on the surface of the globe are engaged in the double task of digging out their channels and carrying away the loose materials that arise from the decomposition of the surface of the land, let us ask ourselves what memorials of these operations they leave behind them. In what form do the running waters of the land inscribe their annals in geological history ? If these waters could suddenly be dried up all over the earth, how could we tell what changes they had once worked upon the surface of the land ? Can we detect the traces of ancient rivers where there are no rivers now ? From what has been said in this lesson it will be evident that Ill EROSION OF CANONS 33 34 RECORDS OF RUNNING WATER CHAP. in answer to such questions as these, we may affirm that one un- mistakable evidence of the former presence of rivers is to be found in the channels which they have eroded. The gorges, rocky defiles, pot-holes, and water-worn rocks which mark the pathway of a stream would long remain as striking memorials of the work of running water. In districts, now dry and barren, such as large regions in the Levant, there are abundant channels (wadies) now seldom or never occupied by a stream, but which were evidently at one time the beds of active torrents. Alluvium. But more universal testimony to the work of running water is to be found in the deposits which it has accumu- lated. To these deposits the general name of alluvium has been given. Spreading out on either side, sometimes far beyond the limits of the ordinary or modern channels, these deposits, even when worn into fragmentary patches, retain their clear record of the operations of the river. Let us in imagination follow the course of a river from the mountains to the sea, and mark as we go the circumstances under which the accumulation of sediment takes place. The power possessed by running water to carry forward sedi- ment depends mainly upon the velocity of the current. The more rapidly a stream flows, the more sediment can it transport, and the larger are the blocks which it can move. The velocity is regulated chiefly by the angle of slope ; the greater the declivity, the higher the velocity and the larger the capacity of the stream to carry down debris. Any cause, therefore, which lessens the velocity of a current diminishes its carrying power. If water, bearing along gravel, sand, or mud, is checked in its flow, some of these materials will drop and remain at rest on the bottom. In the course of every stream, various conditions arise whereby the velocity of the current is reduced. One of the most obvious of these is a diminution in the slope of the channel. Another is the union of a rapid tributary with a more gently flowing stream. A third is the junction of a stream with the still waters of a lake (see p. 42) or with the sea. In these circumstances, the flow of the water being checked, the sediment at once begins to fall to the bottom. Tracing now the progress of a river, for illustrations of this law of deposition, we find that among the mountains where the river takes its rise, the torrents that rush down the declivities have torn out of them such vast quantities of soil and rock as to seam them with deep clefts and gullies. Where each of these rapid stream- lets reaches the valley below, its rapidity of motion is at once Ill ALLUVIAL DEPOSITS 35 lessened, and with this slackening of speed and consequent loss of carrying power, there is an accompanying deposit of detritus. Blocks of rock, angular rubbish, rounded shingle, sand, and earth are thrown down in the form of a cone of which the apex starts from the bottom of the gully and the base spreads out over the plain (Fig. 10). Such cones vary in dimensions according to the FJG. 10. Gullies torn out of the side of a mountain by descending torrents, with cones of detritus at their base. size of the torrent and the comparative ease with which the rocks of the mountain-side can be loosened and removed. Some of them, thrown down by the transient runnels of the last sudden rain-storm, may not be more than a few cubic yards in bulk. But on the skirts of mountainous regions they may grow into masses hundreds of feet thick and many miles in diameter. The valleys in a range of mountains afford many striking examples of these alluvial cones or -fans, as they are called. Where the tributary 36 RECORDS OF RUNNING WATER CHAP. torrents are numerous, a succession of such cones or fans, nearly or quite touching each other, spreads over the floor of a valley. From this cause, so large an amount of detritus has within historic times been swept down into some of the valleys of the Tyrol that churches and other buildings are now half-buried in the accumulation. Looking more closely at the materials brought down by the torrents, we find them arranged in rude irregular layers, sloping downwards into the plain, the coarsest and most angular detritus lying nearest to the mountains, while more rounded and water-worn shingle or sand extends to the outer margin of the cone. This grouping of irregular layers of angular and half-rounded detritus is characteristic of the action of torrents. Hence, where it occurs, even though no water may run there at the present day, it may be regarded as indicating that at some former time a torrent swept down detritus over that site. Quitting the more abrupt declivities, and augmented by numerous tributaries from either side, the stream whose course we are tracing loses the character of a torrent and assumes that of a river. It still flows with velocity enough to carry along not only mud and sand, but even somewhat coarse gravel. The large angular blocks of the torrential part of its course, however, are no longer to be seen, and all the detritus becomes more and more rounded and smoothed as we follow it towards the plains. At many places, deposits of gravel or sand take place, more especi- ally at the inner side of the curves which the stream makes as it winds down the valley. Sweeping with a more rapid flow round the outer side of each curve, the current lingers in eddies on the inner side and drops there a quantity of sediment. When the water is low, strips of bare sand and shingle on the concave side of each bend of the stream form a distinctive feature in river scenery. It is interesting to walk along one of these strips and to mark how the current has left its record there. The stones are well smoothed and rounded, showing that they have been rolled far enough along the bottom of the channel to lose their original sharp edges, and to pass from the state of rough angular detritus into that of thoroughly water-worn gravel. Further, they will be found not to lie entirely at random, as might at first sight be imagined. A little examination will show that, where the stones are oblong, they are generally placed with their longer axis pointing across the stream. This would naturally be the position which they would assume where the current kept Ill ARRANGEMENT OF ALLUVIUM 37 rolling them forward along the channel. Those which are flat in shape will be observed usually to slope up stream. That the sloping face must look in the direction from which the current moves will be evident from Fig. u, where a current, moving in the direction of the arrow and gradually diminishing in force, would no longer be FIG. n. Flat stones in a bank of river-shingle, showing able to Overturn the the direction of the current (indicated by the arrow) ...... . that transported and left them. stones which it had so placed as to offer the least obstacle to its passage. Had the current flowed from the opposite quarter, it would have found the upturned edges of the stones exposed to it, and would have readily overturned them until they found a position in which they again presented least resistance to the water. In a section of gravel, it is thus often quite possible to tell from what quarter the current flowed that deposited the pebbles. Yet another feature in the arrangement of the materials is well seen where a digging has been made in one of the alluvial banks, but better still in a section of one of the terraces to be immediately referred to. The layers of gravel or sand in some bands may be observed to be inclined at a steeper angle than in others, as shown in the accompany- ing figure (Fig. 12). In such cases, it will be noticed that the slope of the more inclined layers is down the stream, and hence that their direction gives a clue to that FIG. 12. Section of alluvium showing direction of currents. of the current which ar- ranged them. We may watch similar layers in the act of deposition among shallow pools into which currents are discharging sediment. The gravel or sand may be observed moving along the bottom, and then falling over the edge of a bank into the bottom of the pool. As the sediment advances by successive additions to its steep slope in front, it gradually fills the pool up. Its progress may be compared to that of a railway embankment formed by the discharge of waggon-loads of rubbish down its end. A section through such an embankment would RECORDS OF RUNNING WATER CHAP. reveal a series of bands of variously coloured materials inclined steeply towards the direction in which the waggon-loads were thrown down. Yet the top of the embankment may be kept quite level for the permanent way. The nearly level bands (, c} in Fig. 12 represent the general bottom on which the sediment accumulated, while the steeper lines in the lower gravel (a) point to the existence and direction of the currents by which sediment was pushed forward along that bottom. (Compare pp. 174, 1750 As the river flows onward through a gradually expanding valley, another characteristic feature becomes prominent. Flank- ing each side of the flat land through which the stream pursues FIG. 13. River-terraces. its winding course, there runs a steep slope or bank a few feet or yards in height, terminating above in a second or higher plain, which again may be bordered with another similar bank, above which there may lie a third plain. These slopes and plains form a group of terraces, rising step by step above and- away from the river, sometimes to a height of several hundred feet, and occasion- ally to the number of 6 or 8 or even more (Fig. 1 3). Here and there, by the narrowing of the intervening strip of plain, two terraces merge into one, and at some places the river in winding down the valley has cut away great slices from the terraces, per- haps even entirely removing them and eating back into the rock out of which the valley has been excavated. Sections are thus exposed showing a succession of gravels, sands, and loams like those of the present river. From the line of the uppermost terrace down to the spits of shingle now forming in the channel, in ORIGIN OF RIVER TERRACES 39 we have evidently a chronologically arranged series of river- deposits, the oldest being at the top and the youngest at the bottom. But how could the river have flowed at the level of these high gravels, so far above its present limits ? An examina- tion of the behaviour of the stream during floods will help towards an answer to this question. When from heavy rains or melted snows the river overflows its banks, it spreads out over the level ground on either side. The tract liable to be thus submerged during inundations is called the flood-plain. As the river rises in flood, it becomes more and more turbid from the quantity of mud and silt poured into it by its tributaries on either side. Its increase in volume likewise augments its velocity, and consequently its power of scouring its bed and of transporting the coarser detritus resting there. Large quantities of shingle may thus be swept out of the ordinary channel and be strewn across the nearer parts of the flood-plain. As the current spreads over this plain, its velocity and transporting capacity diminish, and consequently sediment begins to be thrown down. Grass, bushes, and trees, standing on the flood-plain, filter some of the sediment out of the water. Fine mud and sand, for instance, adhere to the leaves and stems, whence they are eventually washed off by rain into the soil under- neath. In this way, the flood-plain is gradually heightened by the river itself. At the same time, the bed of the river is deepened by the scour of the current, until, in the end, even the highest floods are no longer able to inundate the flood-plain. The differ- ence of level between that plain and the surface of the river gradually increases ; by degrees the river begins to cut away the edges of the terrace which it cannot now overflow, and to form a new flood-plain at a lower level. In this manner, it slowly lowers its bed, and leaves on either side a set of alluvial terraces to mark successive stages in the process of excavation. If during this process the level of the land should be raised, the slope of the rivers, and consequently their scour, would be augmented, and they would thereby acquire greater capacity for the formation of terraces. There is reason to believe that this has taken place both in Europe and North America. While it is obvious that the highest terraces must be the oldest, and that the series is progressively younger down to the terrace that is being formed at the present time, nevertheless, in the materials comprising any one terrace, those lying at the top must be the youngest. This apparent contradiction arises from 40 RECORDS OF RUNNING WATER CHAP. the double action of the river in eroding its bed and depositing its sediment. If there were no lowering of the channel, then the deposits would follow the usual order of sequence, the oldest being below and the youngest above. This order is maintained in the constituents of each single terrace, for the lowermost layers of gravel must evidently have been accumulated before the deposit of those that overlie them. But when the level of the water is lowered, the next set of deposits must, though younger, lie at a lower level than those that preceded them. In no case, however, will the older beds, though higher in position, be found really to overlie the younger. They have been formed at different levels. The gravel, sand, and loam laid down by a river are marked, as we have seen, by an arrangement in layers, beds, or strata lying one upon another. . This stratified disposition indeed is characteristic of all sedimentary accumulations, and is best developed where currents have been most active in transporting and assorting the materials (p. 172). It is the feature that first catches the eye in any river-bank, where a section of the older deposits or " alluvium " is exposed. Beds of coarser and finer detritus alternate with each other, but the coarsest are generally to be observed below and the finest above. The " deltas " accu- mulated by rivers in lakes and in the sea will be noticed in Chapters IV and VII. But besides the inorganic detritus carried forward by a river, we have also to consider the fate of the remains of plants and the carcases of animals that are swept down, especially during floods. Swollen by sudden and heavy rains, a river will rise above its ordinary level and uproot trees and shrubs. On such occasions, too, moles and rabbits are drowned and buried in their burrows on the alluvial flood-plain. Birds, insects, and even some of the larger mammals are from time to time drowned, swept away by floods and buried in the sediment, and their remains, where of a durable kind or where sufficiently covered over, may be preserved for an indefinite period. The shells and fishes living in the river itself may also be killed during the flood, and may be entombed with the other organisms in the sediment. Summary. The material produced by the universal decay of the surface of the land is washed off by rain and swept seawards by brooks and rivers. The rate at which the general level of the land is being lowered by the operation of running water may be approximately ascertained by measuring or estimating the amount of mineral matter carried seaward every year from a definite region, in SUMMARY 4I such as a river-basin. Taking merely the matter in mechanical suspension, and assuming that the proportion of it transported annually in the water of the Mississippi may be regarded as an average proportion for the rivers of Europe, we find that this continent, at the Mississippi rate of degradation, might be reduced to the sea-level in rather less than 6,000,000 years. In pursuing their course over the land, running waters gradu- ally deepen and widen the channels in which they flow, partly by chemically dissolving the rocks and partly by rubbing them down by the friction of the transported sand, gravel, and stones. When they have once chosen their channels, they usually keep to them, and the sinuous windings, at first determined by trifling inequalities on the surface of country across which the streams began to flow, are gradually deepened into picturesque gorges. In the excavation of such ravines, waterfalls play an important part by gradually receding up stream. River-channels, especially if cut deeply into the solid rock, remain as enduring monuments of the work of running water. But still more important as geological records, because more frequent and covering a larger area, are the deposits which rivers leave as their memorials. Whatever checks the velocity of a current weakens its transporting power, and causes it to drop some of its sediment to the bottom. Accordingly, accumulations of sediment occur at the foot of torrent slopes, along the lower and more level ground, especially on the inner or concave side of the loops, over the flood-plains, and finally in the deltas formed where rivers enter lakes or the sea. In these various situations, thick stratified beds of silt, sand, and gravel may be formed, enclosing the remains of the plants and animals living on the land at the time. As a river deepens its channel, it leaves on either side alluvial terraces that mark successive flood-plains over which it has flowed. CHAPTER IV THE MEMORIALS LEFT BY LAKES Fresh-water Lakes. According to the law stated in last chapter, that when water is checked in its flow, it must drop some of its sediment, lakes are pre-eminently places for the deposition and accumulation of mineral matter. In their quiet depths, the debris worn away from the surface of the land is filtered out of the water and allowed to gather undisturbed upon the bottom. The tributary streams may enter a large lake swollen and muddy, but the escaping river is transparent. It is evident, therefore, that lakes must be continually silting up, and that when this process is complete, the site of a lake will be occupied by a series of deposits comprising a record of how the water was made to disappear. To those who know the aspect of lakes only in fine weather, they may seem places where geological operations are at their very minimum of activity. The placid surface of the water ripples upon beaches of gravel or spits of sand ; reeds and marshy plants grow out into the shallows ; the few streamlets that tumble down from the surrounding hills furnish perhaps the only sounds that break the stillness, but their music and motion are at once hushed when they lose themselves in the lake. The scene might serve as the very emblem of perfectly undisturbed conditions of repose. But come back to this same scene during an autumn storm, when the mists have gathered all round the hills, and the rain, after pouring down for hours, has turned every gully into the track of a roaring torrent. Each tributary brook, hardly visible perhaps in drought, now rushes foaming and muddy from its dell and sweeps out into the lake. The large streams bear along on their swift brown currents trunks of trees, leaves, twigs, with now and then the carcase of some animal that has been drowned by the rising CHAP. IV SILTING UP OF LAKES 43 flood. Hour after hour, from every side, these innumerable swollen waters bear their freights of gravel, sand, and mud into the lake. Hundreds or thousands of tons of sediment must thus be swept down during a single storm. When we multiply this result by the number of storms in a year and by the number of years in an ordinary human life, we need not be surprised to be told that even within the memory of the present generation, and still more within historic times, conspicuous changes have taken place in many lakes. Filling up of Lakes. In the Lake of Lucerne, for example, the River Reuss, which bears down the drainage of the huge mountains round the St. Gothard, deposits about 7,000,000 cubic feet of sediment every year. Since the year 1714 the Kander, FIG. 14. Alluvial terraces on the side of an emptied reservoir. which drains the northern flanks of the centre of the Bernese Oberland, is said to have thrown into the lower end of the Lake of Thun such an amount of sediment as to form an area of 230 acres, now partly woodland, partly meadow and marsh. Since the time of the Romans, the Rhone has filled up the upper end of the Lake of Geneva to such an extent that a Roman harbour, still called Port Valais, is now nearly two miles from the edge of the lake, the intervening ground having been converted first into marshes and then into meadows and farms. It is at the mouths of streams pouring into a lake that the process of filling up is most rapid and striking. But it may be detected at many other places round the margin. Instructive lessons on this subject may be learned at a reservoir formed by damming back the waters of a steep-sided valley, and liable to be sometimes nearly dry (Fig. 14). In such a situation, when the water is low, it may be noticed that a series of parallel lines runs all round the sides of the reservoir, and that these lines 44 RECORDS OF LAKES CHAP. consist of gravel, sand, or earth. Each of them marks a former level of the water, and they show that the reservoir was not drained off at once but intermittently, each pause in the diminution of level being marked by a line of sediment. It is easy to watch how these lines are formed along the present margin of the water. The loose debris from the bare slope above, partly by its own gravitation, partly by the wash of rain, slides down into the water. But as soon as it gets there, its further downward movement is FIG. 15. Parallel roads of Glen Roy. arrested. By the ripple of the water it is gently moved up and down, but keeps on the whole just below the line to which the water reaches. So long as it is concealed under the water, its position and extent can hardly be realised. But as soon as the level of the reservoir sinks, the sediment is left as a marked shelf or terrace. In natural lakes, the same process is going on, though hardly recognisable, because concealed under the water. But if by any means a lake could be rapidly emptied, its former level would be marked by a shelf or alluvial terrace. In some cases, the barrier of a lake has been removed, and the sinking of the water has revealed the terrace. The famous " parallel roads " of Glen Roy, in the west of Scotland, are notable examples (Fig. 1 5). IV SILTING UP OF LAKES 45 The valleys in that region were anciently dammed up by large glaciers. The drainage accumulated behind the ice, filled up the valleys and converted them into a series of lakes or fresh-water "fjords." The former levels of these sheets of water and the successive stages of their diminution and disappearance are shown by the series of alluvial shelves known as "parallel roads." The highest of these is 1 140 feet, the middle 1059 feet, and the lowest 847 feet above the level of the sea. Thus, partly by the washing of detritus down from the adjoin- ing slopes by rain, partly by the sediment carried into them by streams, and partly by the growth of marshy vegetation along their margins, lakes are visibly diminishing in size. In mountain- ous countries, every stage of this appearance may be observed (Fig. 1 6). Where the lakes are deep, the tongues of sediment or FIG. 16. Stages in the filling up of a lake. In A two streamlets are represented as pouring their "deltas" into a lake. In B they have filled the lake up, converting it into a meadow across which they wind on their way down the valley. " deltas " which the streams push in front of them have not yet been able to advance far from the shore. In other cases, every tributary has Jpuilt up an alluvial plain which grows outwards and along the coast, until it unites with those of its neighbours to form a nearly continuous belt of flat meadow and marsh round the lake. ' By degrees, as this belt increases in width, the lake narrows, until the whole tract is finally converted into an alluvial plain, through which the river and its tributaries wind on their way to lower levels. The successive flat meadow-like expansions of valleys among hills and mountains were probably in most cases originally lakes which have in this manner been gradually filled up. Lake Deposits. The bottoms of lakes must evidently con- tain many interesting relics. Dispersed through the shingle, sand, and mud that gather there, are the remains of plants and animals that lived on the surrounding land. Leaves, fruits, twigs, RECORDS OF LAKES CHAP. branches, and trunks embedded in the silt may preserve for an indefinite period their record of the vegetation of the time. The wings or wing-cases of insects, the shells of land-snails, the bones of birds and mammals, carried down into the depths of a lake and entombed in the silt there, will remain as a chronicle of the kind of animals that haunted the surrounding hills and valleys. The layers of gravel, sand, and silt laid down on the floor of a lake differ in some respects from those deposited in the terraces of a river, being generally finer in grain, and including a larger proportion of silt, mud, or clay among them, especially away from the margin of the lake. They are, no doubt, further distinguished by the greater abundance of the remains of plants and animals preserved in them. But lakes likewise serve as receptacles for a series of deposits which are peculiar to them, and which consequently have much interest and importance as they furnish a ready means of detecting the sites of lakes that have long disappeared. The molluscs that live in lacustrine waters are distinct from the snails of the adjoin- ing shores. Their dead shells gather on the bottoms of some lakes in such numbers as to form there a deposit of the white crumbling marl, already referred to on p. 4. In course of time this deposit may grow to be many feet or yards in thickness. The shells in the upper parts may be quite fresh, some of the animals having only recently died ; but they become more and more decayed below until, towards the bottom of the deposit, the marl passes into a more com- pact chalk-like substance in which few or no shells maybe recognisable (Fig. 17). On the sites of lakes that have been naturally filled up or artificially drained, such marl has been extensively dug as a manure for land. Besides the shells from the decay of which it is chiefly formed, it sometimes yields the bones of deer, oxen, and other animals, whose carcases must originally have sunk to the bottom of the lake and been there gradually covered up in the growing mass of marl. Many FIG. 17. Piece of shell-marl containing shells of Limncea. peregra. iv SALT-LAKES 47 examples of these marl-deposits are to be found among the drained lakes of Scotland and Ireland. Yet another peculiar accumulation is met with on the bottom of some lakes, particularly in Sweden. In the neighbourhood of banks of reeds and on the sloping shallows of the larger lakes, a deposit of hydrated peroxide of iron takes place, in the form of concretions varying in size from small grains like gunpowder up to cakes measuring six inches across. The iron is no doubt dis- solved out of the rocks of the neighbourhood by water containing organic acids or carbonic acid. In this condition, it is liable to be oxidised on exposure. As after oxidation it can no longer be retained in solution, it is precipitated to the bottom where it collects in grains which by successive additions to their surface become pellets, balls, or cakes. Possibly some of the microscopic plants (diatoms) which abound on the bottoms of the lakes may facilitate the accumulation of the iron by abstracting this substance from the water and depositing it inside their siliceous coverings. Beds of concretionary brown ironstone are formed in Sweden from 10 to 200 yards long, 5 to i 5 yards broad, and from 8 to 30 inches thick. During winter when the lakes are frozen over, the iron is raked up from the bottom through holes made for the purpose in the ice, and is largely used for the manufacture of iron in the Swedish furnaces. When the iron has been removed, it begins to form again, and instances are known where, after the supply had been completely exhausted, beds several inches in thickness were formed again in twenty-six years. Salt-Lakes. The salt-lakes of desert regions present a wholly peculiar series of deposits. These sheets of water have no outlet ; yet there is reason to believe that most of them were at first fresh, and discharged their outflow like ordinary lakes. Owing to geological changes of level and of climate, they have long ceased to overflow. The water that runs into them, instead of escaping by a river, is evaporated back into the air. But the various mineral salts carried by it in solution from rocks and soils are not evaporated also. They remain behind in the lakes, which are consequently becoming gradually salter. Among the salts thus introduced, common salt (sodium-chloride) and gypsum (calcium- sulphate) are two of the most important. These substances, as the water evaporates in the shallows, bays, and pools, are precipitated to the bottom where they form solid layers of salt and gypsum. The latter substance begins to be thrown down when 37 per cent of the water containing it has been evaporated. The sodium-chloride 48 RECORDS OF LAKES CHAP, iv does not appear until 93 per cent of the water has disappeared. In the order of deposit, therefore, gypsum comes before the salt (see p. 136). Some bitter lakes contain sodium-carbonate, in others magnesium -chloride is abundant. The Dead Sea, the Great Salt Lake of Utah, and many other salt lakes and inland seas furnish interesting evidence of the way in which they have gradually changed. In the upper terraces of the Great Salt Lake, 1000 feet or more above the present level of the water, fresh-water shells occur, showing that the basin was at first fresh. The valley-bottoms around saline lakes are now crusted with gypsum, salt, or other efflorescence, and their waters are almost wholly devoid of life. Such conditions as these help us to understand how great deposits of gypsum and rock-salt were formed in England, Germany, and many other regions where the climate would not now permit of any such condensation of the water (Chapter XXII). Summary. The records inscribed by lakes in geological history consist of layers of various kinds of sediment. These deposits may form mere shelves or terraces along the margin of the water which, if drained off, will leave them as evidence of its former levels. By the long-continued operations of rain, brooks, and rivers, continually bringing down sediment, lakes are gradually filled up with alluvium, and finally become flat meadow-land with tributary streams winding through it. The deposits that thus replace the lacustrine water consist mainly of sand or gravel near shore, while finer silt occupies the site of the deeper water. They may also include beds of marl formed of fresh-water shells, and sheets of brown iron ore. Throughout them all, remains of the plants and animals of the surrounding land are likely to be entombed and preserved. Salt lakes leave, as their enduring memorial, beds of rock-salt / and gypsum, sometimes carbonate of soda and other salts. Many of them were at first fresh, as is shown by the presence of ordinary fresh-water shells in their upper terraces. But by change of climate and long-continued excess of evaporation over precipitation, the water has gradually become more and more saline, and has sometimes disappeared altogether, leaving behind it deposits of common salt, gypsum, and other chemical precipitates. CHAPTER V HOW SPRINGS LEAVE THEIR MARK IN GEOLOGICAL HISTORY THE changes made by running water upon the land are not con- fined to that portion of the rainfall which courses along the surface. Even when it sinks underground and seems to have passed out of the general circulation, the subterranean moisture does not remain inactive. After travelling for a longer or shorter distance through the pores of rocks, or along their joints and other divisional planes, it finds its way once more to daylight and reappears in Springs.^ In this underground journey, it corrodes rocks, somewhat in the same way as rain attacks those that are exposed to the outer air, and it works some curious changes upon the face of the land. Subterranean water thus leaves distinct and characteristic memorials as its contribution to geological history. There are two aspects in which the work of underground water may be considered here. In the first place, portions of the sub- stance of subterraneari rocks are removed by the percolating water and in large measure carried up above ground ; in the second place, some of these materials are laid down again in a new form and take a conspicuous place among the geological monuments of their time. In the removal of mineral substance, water percolating through rocks acts in two distinct ways, mechanical and chemical, each of which shows itself in its own peculiar effects upon the surface. (i) Mechanical Action. While slowly filtering through porous materials, water tends to remove loose particles and thus to lessen the support of overlying rocks. But even where there is no transport, the water itself, by saturating a porous layer that rests upon a more or less impervious one, loosens the cohesion of that porous layer. The overlying mass of rock is thus made to 1 Physical Geography Class-Book, p. 222. E 50 RECORDS LEFT BY SPRINGS CHAP. rest upon a watery and weakened platform, and if from its position it should have a tendency to gravitate in any given direction, it may at last yield to this tendency and slide downwards. Along the sides of sea-cliffs, on the precipitous slopes of valleys or river- gorges, or on the declivities of hills and mountains, the conditions are often extremely favourable for the descent of large masses, of rock from higher to lower levels. Remarkable illustrations of such Landslips, as they are called, have been observed along the south coast of England, where certain porous sandy rocks underlying a thick sheet of chalk rest upon more or less impervious clays, which, by arresting the water in its descent, throw it out along the base of the slopes. After much wet weather, the upper surface of these clays becomes, as it were, lubricated by the accumulation of water, and large slices of the overlying rocks, having their support thereby weakened, break off from the solid cliffs behind and slide down towards the sea. The most memorable example occurred at Christmas time, in the year 1839, on the coast of Devonshire not far from Axmouth. At that locality, the chalk-downs end off in a line of broken cliff some 500 feet above the sea. From the edge of the downs flanked by this cliff a tract about 800 yards long, containing not less than 30 acres of arable land, sank down with all its fields, hedgerows, and path- ways. This sunken mass, where it broke away from the upland, left behind it a new cliff, showing along the crest the truncated ends of the fields, of which the continuation was to be found in a chasm more than 200 feet deep. While the ground sank into this defile and was tilted steeply towards the base of the cliff, it was torn up by a long rent running on the whole in the line of the cliff, and by many parallel and transverse fissures. Half a century has passed away since this landslip occurred. The cliff remains much as it was at first, and the sunken fields with their bits of hedgerow still slope steeply down to the bottom of the declivity (Fig. 1 8). But the lapse of time has allowed the influence of the atmosphere to come into play. The outstanding dislocated frag- ments with their vertical walls and flat tops, showing segments of fields, have been gradually worn into tower-like masses with sloping declivities of debris. The long parallel rent has been widened by rain into a defile with shelving sides. Everywhere the rawness of the original fissures has been softened by the rich tapestry of verdure which the genial climate of that southern coast fosters in every sheltered nook. But the scars have not been healed, and they will no doubt remain still visible for many 5. year to come. LANDSLIPS 52 RECORDS LEFT BY SPRINGS CHAP. Along the south coast of England, many landslips, of which there is no historical record, have produced some of the most picturesque scenery of that region. Masses that have slipped away from the main cliff have so grouped themselves down the slopes that hillocks and hollows succeed each other in endless confusion, as in the well-known Undercliff of the Isle of Wight. Some of the tumbled rocks are still fresh enough to show that they have fallen at no very remote period, or even that the slipping still continues ; others, again, have yielded so much to the weather that their date doubtless goes far back into the past, and some of them are crowned with what are now venerable ruins. The most stupendous landslips on record have occurred in mountainous countries. Upwards of 150 destructive examples have been chronicled in Switzerland. Of these, one of the most memorable was that of the Rossberg, a mountain lying behind the Rigi, and composed of thick masses of hard red sandstone and conglomerate so arranged as to slope down into the valley of Goldau. The summer of the year 1 806 having been particularly wet, so large an amount of water had collected in the more porous layers of rock as to weaken the support of the overlying mass ; consequently a large part of the side of the mountain suddenly gave way and rushed down into the valley, burying under the debris about a square German mile of fertile land, four villages containing 330 cottages and outhouses, and 457 inhabitants. To this day, huge angular blocks of sandstone lying on the farther side of the valley bear witness to the destruction caused by this land- slip, and the scar on the mourfPain-slope whence the fallen masses descended is still fresh. ^2) Chemical Action (a) Solution. But it is by its chemi- cal action on the rocks through which it flows that subterranean water removes by far the largest amount of mineral matter, and produces the greatest geological change. Even pure water will dissolve a minute quantity of the substance of many rocks. But rain is far from being chemically pure water. In previous chapters it has been described as taking oxygen and carbonic acid out of the air in its descent, and abstracting organic acids and carbonic acid from the soil through which it sinks. By help of these in- gredients, it is enabled to attack even the most durable rocks, and to carry some of their dissolved substance up to the surface of the ground. One of the substances most readily attacked and removed even by pure water is the mineral known as carbonate of lime. Among v CHEMICAL ACTION OF SPRINGS 53 other impurities, natural waters generally contain carbonic acid, which may be derived from the air or from the soil ; occasionally from some deeper subterranean source. The presence of this acid gives the water greatly increased solvent power, enabling it readily to attack carbonate of lime, whether in the form of limestone, or diffused through rocks composed mainly of other substances. Even lime, which is not in the form of carbonate, but is united with silica in various crystalline minerals (silicates, p. 130), may by this means be decomposed and combined with carbonic acid. It is then removed in solution as carbonate. So long as the water retains enough of free carbonic acid, it can keep the car- bonate of lime in solution and carry it onward. Limestone is a rock almost entirely composed of carbonate of lime. It occurs in most parts of the world, covering sometimes tracts of hundreds or thousands of square miles, and often rising into groups of hills, or even into ranges of mountains (see pp. 154, 158). The abundance of this rock affords ample opportunity for the display of the solvent action of subterranean water. Trick- ling down the vertical joints and along the planes between the limestone beds, the water dissolves and removes the stone, until in the course of centuries these passages are gradually enlarged into clefts, tunnels, and caverns. The ground becomes honey- combed with openings into dark subterranean chambers, and running streams fall into these openings and continue their course underground. Every country which possesses large limestone tracts furnishes examples of the way in which such labyrinthine tunnels and systems of caverns are excavated. In England, for example, the Peak Cavern of Derbyshire is believed to be 2300 feet long, and in some places 120 feet high. On a much more magnificent scale are the caverns of Adelsberg near Trieste, which have been ex- plored to a distance of between four and five miles, but are probably still more extensive. The river Poik has broken into one part of the labyrinth of chambers, through which it rushes before emerg- ing again to the light. Narrow tunnels expand into spacious halls, beyond which egress is again afforded by low passages into other lofty recesses. The most stupendous chamber measures 669 feet in length, 630 feet in breadth, and 1 1 1 feet in height. From the roofs hang pendent white stalactites (p. 55), which, uniting with the floor, form pillars of endless varieties of. form and size. Still more gigantic is the system of subterranean passages in the Mammoth Cave of Kentucky, the accessible parts of which are 54 RECORDS LEFT BY SPRINGS CHAP. believed to have a combined length of about 150 miles. The largest cavern in this vast labyrinth has an area of two acres, and is covered by a vault 125 feet high. Of the mineral matter dissolved by permeating water out of the rocks underground, by far the larger part is discharged by springs into rivers, and ultimately finds its way to the sea. The total amount of material thus supplied to the sea every year must be enormous. Much of it, indeed, is abstracted from ocean-water by the numerous tribes of marine plants and animals. In particular, the lime, silica, and organic matter are readily seized upon to build up the framework and furnish the food of these creatures. But probably more mineral matter is supplied in solution than is re- FIG. 19. Section of cavern with stalactites and stalagmite. quired by the organisms of the sea, in which case the water of the sea must be gradually growing heavier and salter. () Deposition. But it is the smaller proportion of the material not conveyed into the sea that specially demands atten- tion. Every spring, even the purest and most transparent, con- tains mineral solutions in sufficient quantity to be detected by chemical analysis. Hence all plants and animals that drink the water of springs and rivers necessarily imbibe these solutions which, indeed, supply some of the mineral salts whereof the harder parts both of plants and animals are constructed. Many springs, however, contain so large a proportion of mineral matter, that when they reach the surface and begin to evaporate, they drop their solutions as a precipitate, which settles down upon the bottom or on objects within reach of the water. After years of undisturbed continuance, extensive sheets of mineral material may in this manner be accumulated, which remain as enduring monu- CALCAREOUS SPRINGS 55 ments of the work of underground water, even long after the springs that formed them have ceased to flow. Calcareous Springs. Among the accumulations of this nature by far the most frequent and important are those formed by what are called Calcareous Springs. In regions abounding in lime- stone or rocks containing much carbonate of lime, the subterranean waters which, as we have seen, gradually erode such vast systems of tunnels, clefts, and caverns, carry away the dissolved rock, and retain it in solution only so long as they can keep their carbonic acid. As soon as they begin to evaporate and to lose some of this acid, they lose also the power of retaining so much carbonate of lime in solution. This substance is accordingly dropped as a fine white precipitate, which gathers on the surfaces over which the water trickles or flows. The most familiar example of this process is to be seen under the arches of bridges and vaults. Long pendent white stalks or stalactites hang from between the joints of the masonry, while wavy ribs of the same substance run down the piers or walls, and even collect upon the ground (stalagmite). A few years may suffice to drape an archway with a kind of fringe of these pencil-like icicles of stone. Per- colating from above through the joints between the stones of the masonry, the rain-water, armed with its minute proportion of carbonic FlG ' '"--Section show- . , , f. _ , ing successive layers or acid at once attacks the lime of the mortar growt h in a stalactite, and forms carbonate of lime, which is carried downward in solution. Arriving at the surface of the arch, the water gathers into a drop, which remains hanging there for a brief interval before it falls to the ground. That interval suffices to allow some of the carbonic acid to escape, and some of the water to evaporate. Consequently, round the outer rim of the drop a slight precipitation of white chalky carbonate of lime takes place. This circular pellicle, after the drop falls, is increased by a similar deposit from the next drop, and thus drop by drop the original rim or ring is gradually lengthened into a tube which may eventually be filled up inside, and may be thickened irregularly outside by the trickle 56 RECORDS LEFT BY SPRINGS CHAP. of calcareous water (Fig. 20). But the deposition on the roof does not exhaust the stock of dissolved carbonate. When the drops reach the ground the same process of evaporation and precipitation continues. Little mounds of the same white substance are built up on the floor, and, if the place remain undisturbed, may grow until they unite with the stalactites from the roof, forming white pillars that reach from floor to ceiling (Fig. 19, and p. 154). It is in. limestone caverns that stalactitic growth is seen on the most colossal scale. These quiet recesses having remained undis- turbed for many ages, the process of solution and precipitation has advanced without interruption until, in many cases, vast caverns have been transformed into grottoes of the most marvel- lous beauty. White glistening fringes and curtains of crystalline carbonate of lime, or spar, as it is popularly called, hang in end- less variety and beauty of form from the roof. Pillars of every dimension, from slender wands up to thick-ribbed columns like those of a cathedral, connect the roof and the pavement. The walls, projecting in massive buttresses and retiring into alcoves, are everywhere festooned with a grotesque drapery of stone. The floor is crowded with mounds and bosses of strangely imitative forms which recall some of the oddest shapes above ground. Wandering through such a scene, the visitor somehow feels him- self to be in another world, where much of the architecture and ornament belongs to styles utterly unlike those which can be seen anywhere else. The material composing stalactite and stalagmite is at first, as already stated, a fine white chalky pulp-like substance which dries into a white powder. But as the deposition continues, the older layers, being impregnated with calcareous water, receive a precipitation of carbonate of lime between their minute pores and crevices, and assume a crystalline structure. Solidifying and hardening by degrees, they end by becoming a compact crystalline stone (spar) which rings under the hammer. The numerous caverns of limestone districts have offered ready shelter to various kinds of wild animals and to man himself. Some of them (Bone-Caves) have been hyaena- dens, and from under their hard floor of stalagmite the bones of hyaenas and of the creatures they fed upon are disinterred in abundance. Rude human implements have likewise been obtained from the same deposits, showing that man was contemporary with animals which have long been extinct. The solvent action of underground water has thus been of the utmost service in geological history, v CALCAREOUS DEPOSITS 57 first, in forming caverns that could be used as retreats, and then in providing a hard incrustation which should effectually seal up and preserve the relics of the denizens left upon the cavern-floors. Calcareous springs, issuing from limestone or other rock abounding in lime, deposit carbonate of lime as a white pre- cipitate. So large is the proportion of mineral contained by some waters that thick and extensive accumulations of it have been formed. The substance thus deposited is known by the name of Calcareous Tufa. Cole-sinter, or Travertine. It varies in texture, some kinds being loose and crumbling, others hard and crystalline. In many places it is composed of thin layers or laminae, of which sixty may be counted in the thickness of an inch, but bound together into a solid stone. These laminae mark the successive layers of deposit. They are formed parallel to the surface over which the water flows or trickles, hence they may be observed not only on the flat bottoms of the pools, but irregularly enveloping the walls of the channel as far up as the dash of water or spray can reach. Rounded bosses may thus be formed above the level of the stream, and the recesses may be hung with stalactites. The calcareous springs of Northern and Central Italy have long been noted for the large amount of their dissolved lime, the rapidity with which it is deposited, and the extensive masses in which it has accumulated. Thus at San Filippo in Tuscany, it is deposited in places at the rate of one foot in four months, and it has been piled up to a depth of at least 250 feet, forming a hill a mile and a quarter long, and a third of a mile broad. So com- pact are many of the Italian travertines that they have from time immemorial been extensively used as a building stone, which can be dressed and is remarkably durable^. Many of the finest build- ings of ancient and modern Rome have been constructed of travertine. A familiar feature of many calcareous springs deserves notice here. The precipitation, of calc-sinter is not always due merely to evaporation. In many cases, where the proportion of carbonate of lime in solution is so small that under ordinary circumstances no precipitation of it would take place, large masses of it have been deposited in a peculiar fibrous form. On examination, this precipitation will be found to be caused by the action of plants, particularly bog-mosses which, decomposing the carbonic acid in the water, cause the lime-carbonate to be deposited along their stems and leaflets. The plants are thus incrusted with sinter 58 RECORDS LEFT BY SPRINGS CHAP. which, preserving their forms, looks as if it were composed -of heaps of moss turned into stone. Hence the name si petrifying springs often given to waters where this process is to be seen. There is, however, no true petrifaction or conversion of the actual substance of the plants into stone. The fibres are merely incrusted with travertine, inside of which they eventually die and decay. But as the plants continue to grow outward, they increase the sinter by fresh layers, while the inner and dead parts of the mass are filled up and solidified by the deposit of the precipitate within their cavities. A growing accumulation of travertine presents a special FIG. 21. Travertine with impressions of leaves. interest to the geologist from the fact that it offers exceptional facilities for the preservation of remains of the plants and animals of the neighbourhood. Leaves from the surrounding trees and shrubs are blown into pools or fall upon moist surfaces where the precipitation of lime is actively going on (Fig. 21). Dead insects, snail-shells, birds, small mammals, and other denizens of the dis- trict may fall or be carried into similar positions. These remains may be rapidly enclosed within the stony substance before they have time to decay, and even if they should afterwards moulder into- dust, the sinter enclosing them retains the mould of their forms, and thus preserves for an indefinite period the record of their former existence. V CHALYBEATE AND SILICEOUS SPRINGS 59 Chalybeate Springs. A second but less abundant deposit from springs is found in regions where the rocks below ground contain decomposing sulphide of iron (p. 137). Water percolating through such rocks and oxidising the sulphur of that mineral, forms sulphate of iron (ferrous sulphate) which it removes in solution. The presence of any notable quantity of this sulphate is at once revealed by the marked inky taste of the water and by the yellowish-brown precipitate on the sides and bottom of the channel. Such water is termed Chalybeate. When it mixes with other water containing dissolved carbonates (which are so generally present in running water), the sulphate is decomposed, the sulphuric acid passing over to the lime or alkali of the carbonate, while the iron takes up oxygen and falls to the bottom as a yellowish-brown precipitate (limonite, p. 129). This interchange of combinations, with the consequent precipitation of iron -oxide, may continue for a considerable distance from the outflow of the chalybeate water. Nearest the source the deposit of hydrated ferric oxide or ochre is thickest. It encloses leaves, stems, and other organic remains, and preserves moulds or casts of their forms. It also cements the loose sand and shingle of a river- bottom into solid rock. Siliceous Springs. One other deposit from spring- water may be enumerated here. In volcanic regions, hot springs (geysers) rise to the surface which, besides other mineral ingredients, con- tain a considerable proportion of silica (p. 117). This substance is deposited as Siliceous Sinter round the vents whence the water is discharged, where it forms a white stone rising into mounds and terraces with fringes and bunches of coral-like growth. Where many springs have risen in the same district, their respect- ive sheets of sinter may unite, and thus extensive tracts are buried under the deposit. In Iceland, for example, one of the sheets is said to be two leagues long, a quarter of a league wide, and a hundred feet thick. In the Yellowstone Park of North America, many valleys are floored over with heaps of sinter, and in New Zealand other extensive accumulations of the same material are to be found. It is obvious that, like travertine, siliceous sinte*r may readily entomb and preserve a record of the plants and animals that lived at the time of its deposition. Summary. The underground circulation of water produces changes that leave durable records in geological history. These changes are of two kinds, (i) Landslips are caused, by which the forms of cliffs, hills, and mountains are permanently altered. 60 RECORDS LEFT BY SPRINGS CHAP, v Vast labyrinths of subterranean tunnels, galleries, and caverns are dissolved out of calcareous rocks, and openings are made from these passages up to the surface whereby rivers are engulfed. Many of the caves thus hollowed out have served as dens of wild beasts and dwelling-places for man, and the relics of these inhabit- ants have been preserved under the stalagmite of the floors. (2) An enormous quantity of mineral matter is brought up to the surface by springs. Most of the solutions are conveyed ulti- mately to the sea where they partly supply the substances required by the teeming population of marine plants and animals. But, under favourable circumstances, considerable deposits of mineral matter are made by springs, more especially in the form of traver- tine, siliceous sinter, and ochre. In these deposits the remains of terrestrial vegetation, also of insects, birds, mammals, and other animals, are not infrequently preserved, and remain as per- manent memorials of the life of the time when they nourished. CHAPTER VI ICE-RECORDS ICE in various ways alters the surface of the land. By disinte- grating and eroding even the most durable rocks, and by removing loose materials and piling them up elsewhere, it greatly modifies the details of a landscape. As it assumes various forms, so it accomplishes its work with considerable diversity. The action of frost upon soil and bare surfaces of rock has already (p. 1 3) been described. We have now to consider the action of frozen rivers and lakes, snow and glaciers, which have each their own char- acteristic style of operation, and leave behind them their distinctive contribution to the geological history of the earth. Frozen Rivers and Lakes. In countries with a severe winter climate, the rivers and lakes are frozen over, and the cake of ice that covers them may be more than two feet thick. When this cake is broken up in early summer, large masses of it are driven ashore, tearing up the littoral boulders, gravel, sand, or mud, and pushing them to a height of many feet above the ordinary level of the water. When the ice melts, huge heaps of detritus are found to have been piled up by it, which remain as enduring monuments of its power. Not only so, but large fragments of the ice that has been formed along shore and has enclosed blocks of stone, gravel, and sand, are driven away and may travel many miles before they melt and drop their freight of stones. On the St. Lawrence and on the coast of Labrador, there is a constant transportation of boulders by this means. Further, besides freezing over the surface, the water not infrequently forms a loose spongy kind of ice on the bottom (Anchor-ice, Ground-ice) which encloses stones and gravel, and carries them up to the surface where it joins the cake of ice there. This bottom-ice is formed abundantly on some parts of the Canadian rivers. Swept down by the current, it accumulates 62 ICE-RECORDS CHAP. against the bars or banks, or is pushed over the upper ice, and from time to time gathers into temporary barriers, the bursting of which may cause destructive floods. In the river St. Lawrence, banks and islets have been to a large extent worn down by the grating of successive ice-rafts upon them. Snow. On level or gently inclined ground, whence snow dis- appears merely by melting or evaporation, it exercises, while it remains, a protective influence upon the soil and vegetation, shielding them from the action of frost. On slopes of suffi- cient declivity, however, the sheet of snow acquires a tendency to descend by gravitation, as we may often see on house-roofs in winter. In many cases, it creeps or slides down the side of a hill or valley, and in so doing pushes forward bare soil, loose stones, or other objects lying on the surface. By this means, the debris of weathered rock in exposed situations is gradually thrust down-hill and the rock is bared for further disintegration. But where the declivities are steep enough to allow the snow to break off in large sheets and to rush rapidly down, the most striking changes are observable. Such descending masses are known as Ava- lanches. Varying from 10 to 50 feet or more in thickness and several hundred yards broad and long, they sweep down the mountain sides with terrific force, carrying away trees, soil, houses, and even large blocks of rock. The winter of 1884-85 was especially remarkable for the number of avalanches in the valleys of the Alps, and for the enormous loss of life and property which they caused. In such mountain ground, not only are declivities bared of their trees, soil, and boulders, but huge mounds of debris are piled up in the valleys below. Frequently, also, such a quantity of snow, ice, and rubbish is thrown across the course of a stream as to dam back the water, which accumulates until it overflows or sweeps away the barrier. In another but indirect way, snow may powerfully affect the surface of a district where, by rapid melting, it so swells the rivers as to give rise to destruc- tive floods. While, therefore, the influence of snow is on the whole to protect the surface of the land, it shows itself in mountainous regions singularly destructive, and leaves as chief memorials of this destructiveness the mounds and rough heaps of earth and stones that mark where the down-rushing avalanches have come to rest. Glaciers and Ice-Sheets leave their record in characters so distinct that they cannot usually be confounded with those of any VI TRANSPORT BY GLACIERS other kind of geological agent. The changes which they produce on the surface of the land may be divided into two parts : ( i ) the transport of materials from the high grounds to lower levels, and (2) the erosion of their beds. (i) Transport. As a glacier descends its valley, it receives upon its surface the earth, sand, mud, gravel, boulders, and blocks of rock that roll or are washed down from the slopes on either side. Most of this rubbish accumulates on the edges of the glacier, where it is slowly borne to lower levels as the ice creeps downwards. But some of it falls into the crevasses or rents by , FIG. 22. Glacier with medial and lateral moraines. which the ice is split, and may either be imprisoned within the glacier, or may reach the rocky floor over which the ice is sliding. The rubbish borne onward upon the surface of the glacier is known as moraine-stuff. The mounds of it running along each side of the glacier form lateral moraines , those on the right-hand side as we look down the length of the valley being the right lateral moraine, those on the other side the left lateral moraine. Where two glaciers unite, the left lateral moraine of the one joins the right lateral moraine of the other, forming what is called a medial moraine that runs down the middle of the united glacier. Where a glacier has many tributaries bearing much moraine-stuff, its ICE-RECORDS CHAP. surface may be like a bare plain covered with earth and stones, so that, except where a yawning crevasse reveals the clear blue gleam of the ice below, nothing but earth and stones meets the eye. When the glacier melts, the detritus is thrown in heaps upon the valley, forming there the terminal moraine. Glaciers, like rivers, are subject to variations of level. Even from year to year they slowly sink below their previous limit or rise above it. The glacier of La Brenva, for example, on the Italian side of Mont Blanc, subsided no less than 300 feet in the first half of the present century. One notable consequence of such diminution is that the blocks of rock lying on the edges of a glacier are stranded on the side of the valley, as the ice shrinks FIG. 23. Perched blocks scattered over ice-worn surface of rock. away from them. Such Perched Blocks or Erratics (Fig. 23), as they are called, afford an excellent means of noting how much higher and longer a glacier has once been than it is now. Their great size (some of them are as large as good-sized cottages) and their peculiar positions make it quite certain that they could not have been transported by any current of water. They are often poised on the tops of crags, on the very edges of precipices, or on steep slopes where they could never have been left by any flood, even had the flood been capable of moving them. The agent that deposited them in such positions must have been one that acted very quietly and slowly, letting the blocks gently sink into the sites they now occupy. The only agent known to us that could have done this is glacier-ice. We can actually see similar blocks on the glaciers now, and others which have only vi ERRATIC BLOCKS 65 recently been stranded on the side of a valley from which the ice has sunk. In the Swiss valleys, the scattered ice-borne boulders may be observed by hundreds far above the existing level of the glaciers and many miles beyond where these now end. If the origin of the dispersed erratics is self-evident in a valley where a glacier is still busy transporting them, those that occur in valleys which are now destitute of glaciers can offer no difficulty ; they become, indeed, striking monuments that glaciers once existed there. Scattered erratic blocks offer much interesting evidence of the movements of the ice by which they were transported. In a glacier-valley, the blocks that fall upon the ice remain on the side from which they have descended. Hence, if there is any notable difference between the rocks of the two sides, this difference will be recognisable in the composition of the moraines, and will remain distinct even to the end of the glacier. If, therefore, in a district from which the glaciers have disappeared, we can trace up the scattered blocks to their sources among the mountains, we thereby obtain evidence of the actual track followed by the vanished glaciers. The limits to which these blocks are traceable do not, of course, absolutely fix the limits of the ice that transported them. They prove, however, that the ice extended at least as far as they occur, but it may obviously have risen higher and advanced farther than the space within which the blocks are now confined. In Europe, some striking examples occur of the use of this kind of evidence. Thus the peculiar blocks of the Valais can be traced all the way to the site of the modern city of Lyons. There can therefore be no doubt that the glacier of the Rhone once extended over all that intervening country and reached at least as far as Lyons, a distance of not less than 1 70 miles from where it now ends. Again, from the occurrence of blocks of some of the char- acteristic rocks of Southern Scandinavia, in Northern Germany, Belgium, and the east of England, we learn that a great sheet of ice once filled up the bed of the Baltic and the North Sea, carrying with it immense numbers of northern erratics. In Britain, where there are now neither glaciers nor snow-fields, the abundant dis- persion of boulders from the chief tracts of high ground shows that this country was once in large part buried under ice, like modern Greenland. The evidence for these statements will be more fully given in a later part of this volume (Chapter XXVII). Besides the moraine-stuff carried along on the surface, loose detritus and blocks of rock are pushed onwards under the ice, F 66 ICE-RECORDS CHAP. When a glacier retires, this earthy and stony debris, where not swept away by the escaping river, is left on the floor of the valley. One remarkable feature of the stones in it is that a large pro- portion of them are smoothed, polished, and covered with fine scratches or ruts, such as would be made by hard sharp-pointed fragments of stone or grains of sand. These markings run for the most part along the length of each oblong stone, but not infrequently cross each other, and sometimes an older may be noticed partially effaced by a newer set. This peculiar striation is a most characteristic mark of the action of glaciers. The stones under the ice are fixed in the line of least resistance that FIG. 24. Stone smoothed and striated by glacier-ice. is, end on. In this position, under the weight of hundreds of feet of ice, they are pressed upon the floor over which the glacier is travelling. Every sharp edge of stone or grain of sand, pressed along the surface of a block, or over which the block itself is slowly drawn, engraves a fine scratch or a deeper rut. As the block moves onward, it is more and more scratched, losing its corners and edges, and becoming smaller and smoother till, if it travel far enough, it may be entirely ground into sand or mud (Fig. 24). (2) Erosion. The same process of erosion is carried on upon the solid rocks over which the ice moves. These are smoothed, striated, and polished by the friction of the grains of sand, pebbles, and blocks of stone crushed against them by the slowly creeping mass of ice. Every boss of rock that looks toward the quarter from which the overlying ice is moving is ground away, while those that face to the opposite side are more or less VI ROCK-STRIATION BY ICE 67 sharp and unworn. The striation is especially noteworthy. From the fine scratches, such as are made by grains of sand, up to deep ruts like those of cart-wheels in unmended roadways, or to still wider and deeper hollows, all the friction -markings run in a general uniform direction, which is that of the motion of the glacier. Such striated surfaces could only be produced by some agent with rigidity enough to hold the sand-grains and stones in position, and press them steadily onward upon the rocks. A 25. Ice-striation on the floor and side of a valley. river polishes the rocks of its channel by driving shingle and sand across them ; but the currents are perpetually tossing these materials now to one side, now to another, so that smoothed and polished surfaces are produced, but with nothing at all resembling striation. A glacier, however, by keeping its grinding materials fixed in the bottom of the ice, engraves its characteristic parallel striae and groovings, as it slowly creeps down the valley. All the surfaces of rock within reach of the ice are smoothed, polished, and striated. Such surfaces present the most unmistakable evidence of glacier-action, for they can be produced by no other known natural agency. Hence, where they occur in glacier 68 ICE-RECORDS CHAP. valleys, far above and beyond the present limits of the ice, they prove how greatly the ice has sunk. In regions also where there are now no glaciers, these rock -markings remain as almost imperishable witnesses that glaciers once existed. By means of their evidence, for example, we can trace the march of great ice- sheets which once enveloped the whole of Scandinavia and lay deep upon nearly the whole of Britain. The river that escapes from the end of a glacier is always milky or muddy. The fine sand and mud that discolour the water are not supplied by the thawing of the clear ice, nor by the sparkling brooks that gush out of the mountain-slopes, nor by the melting of the snows among the peaks that rise on either side. This material can only come from the rocky floor of the glacier itself. It is the fine sediment ground away from the rocks and loose stones by their mutual friction under the pressure of the overlying ice. It serves thus as a kind of index or measure of the amount of material worn off the rocky bed by the grinding action of the glacier. We can readily see that as this erosion and transport are continually in progress, the amount of material removed in the course of time must be very great. It has been estimated, for example, that the Justedal glacier in Norway removes annually from its bed 2,427,000 cubic feet of sediment. At this rate the amount removed in a century would be enough to fill up a valley or ravine 10 miles long, 100 feet broad, and 40 feet deep. In arctic and antarctic latitudes, where the land is buried under a vast ice-sheet, which is continually creeping seaward and break- ing off into huge masses that float away as icebergs, there must be a constant erosion of the terrestrial surface. Were the ice to retire from these regions, the ground would be found to wear what is called a glaciated surface ; that is to say, all the bare rocks would present a characteristic ice-worn aspect, rising into smooth rounded bosses like dolphins' backs (roches ?noutonne'es\ and sinking into hollows that would become lake-basins. Every- where these bare rocks would show the striae and groovings graven upon them by the ice, radiating generally from the central high grounds, and thus indicating the direction of flow of the main streams of the ice-sheet. Piles of earth, ice-polished stones, and blocks of rock would be found strewn over the country, especially in the valleys and over the plains. These materials would still further illustrate the movements of the ice, for they would be found to be singularly local in character, each district having vi SUMMARY 69 supplied its own contribution of detritus. Thus from a region of red sandstone, the rubbish would be red and sandy ; from one of black slate, it would be black and clayey (see Chapter XXVI). Summary. In this chapter we have seen that Ice in various ways affects the surface of the land and leaves its mark there. Frost, as already explained in Chapter II, pulverises soil, dis- integrates exposed surfaces of stone, and splits open bare rocks along their lines of natural joint. On rivers and lakes, the disrupted ice wears down banks and pushes up mounds of sand, gravel, and boulders along the shores. Snow lying on the surface of the land protects that surface from the action of frost and air. In the condition of avalanches, snow causes large quantities of earth, soil, and blocks of rock to be removed from the mountain- slopes and piled up on the valleys. In the form of glaciers, ice transports the debris of the mountains to lower levels, bearing along and sometimes stranding masses of rock as large as cottages, which no other known natural agent could transport. Moving down a valley, a glacier wears away the rocks, giving them a peculiar smoothed and striated surface which is thoroughly characteristic. By this grinding action, it erodes its bed and produces a large amount of fine sediment, which is carried away by the river that escapes at the end of the ice-stream. Land-ice thus leaves thoroughly distinctive and enduring memorials of its presence in polished and grooved rocks, in masses of earth, clay, or gravel, with striated stones, and in the dispersal of erratic blocks from principal masses of high ground. These memorials may remain for ages after the ice itself has vanished. By their evidence we know that the present glaciers of the Alps are only a shrunk remnant of the great ice-fields which once covered that region ; that the Scandinavian glaciers swept across what is now the bed of the North Sea as far as the mouth of the Thames ; and that Scotland, Ireland, Wales, and the greater part of England were buried under great sheets of ice which crept downwards into the North Sea on the one side, and into the Atlantic on the other (Chapter XXVII). CHAPTER VII THE MEMORIALS OF THE PRESENCE OF THE SEA WE have now to inquire how the work of the Sea is registered in geological history. This work is broadly of two kinds. In the first place, the sea is engaged in wearing away the edges of the land, and in the second place, being the great receptacle into which all the materials, worn away from the land, are transported, it arranges these materials over its floor, ready to be raised again into land at some future time. i. Demolition of the Land. In its work of destruction along the coasts of the land, the sea acts to some extent (though we do not yet know how far) by chemically dissolving the rocks and sediments which it covers. Cast-iron bars, for example, have been found to be so corroded by sea-water as to lose nearly half their strength in fifty years. Doubtless many minerals and rocks are liable to similar attacks. But it is by its mechanical effects that the sea accomplishes most of its erosion. The mere weight with which ocean-waves fall upon exposed coasts breaks off fragments of rock from cliffs. Masses, 1 3 tons in weight, have been known to be quarried out of the solid rock by the force of the breakers in Shetland, at a height of 70 feet above sea-level. As a wave may fall with a blow equal to a pressure of 3 tons on the square foot, it compresses the air in every cleft and cranny of a cliff, and when it drops it allows the air instantly to expand again. By this alternate compression and expansion, portions of the cliff are loosened and removed. Where there is any weaker part in the rock, a long tunnel may be exca- vated, which may even be drilled through to the daylight above, forming an opening at some distance inland from the edge of the cliff. During storms, the breakers rush through such a tunnel, and spout forth from the opening (or blow-hole) in clouds of spray (Fig. 26). CHAP. VII ACTION OF BREAKERS Probably the most effective part of the destructive action of the sea is to be found in the battery of gravel, shingle, and loose blocks of stone which the waves discharge against cliffs exposed to their fury. These loose materials, caught up by the advancing breakers and thrown with great force upon the rocks of a coast-line, are dragged back in the recoil of the water, but only to be again lifted and swung forward. In this loud turmoil, the loose stones are reduced in size and are ground smooth by friction against each FIG. 26. Buller of Buchan a caldron-shaped cavity or blow-hole worn out of granite by the sea on the coast of Aberdeenshire. other and upon the solid cliff. The well-rounded and polished aspect of the gravel on such storm-beaten shores is an eloquent testimony to the work of the waves. But still more striking, because more measurable, is the proof that the very cliffs them- selves cannot resist the blows dealt upon them by the wave-borne stones. Above the ordinary limit reached by the tides, the rocks rise with a rough ragged face, bearing the scars inflicted on it by the ceaseless attacks of the air, rain, frost, and the other agencies that waste the surface of the land. But all along the base of the cliff, within reach of the waves, the rocks have been smoothed and 72 MEMORIALS LEFT BY THE SEA CHAP. polished by the ceaseless grinding of the shingle upon them, while arches, tunnels, solitary pillars, half-tide skerries, creeks, and caves attest the steady advance of the sea and the gradual demolition of the shore. Every rocky coast-line exposed to a tempestuous sea affords illustrations of these features of the work of waves. Even where the rocks are of the most durable kind, they cannot resist the ceaseless artillery of the ocean. They are slowly battered down, and every stage in their demolition may be witnessed, from the sunken reef, which at some distance from the shore marks where the coast-line once ran, up to the tunnelled cliff from which a huge mass was detached during the storms of last winter. But where the materials composing the cliffs are more easily removed, the progress of the waves may be comparatively rapid. Thus on the east coast of Yorkshire between Spurn Point and Flamborough Head, the cliffs consist of boulder-clay, and vary up to more than 100 feet in height. At high water, the tide rises against the base of these cliffs, and easily scours away the loose debris which would otherwise gather there and protect them. Hence, within historic times, a large tract of land, with its parishes, farms, villages, and sea- ports, has been washed away, the rate of loss being estimated at not less than 2^ yards in a year. Since the Roman occupation a strip of land between 2 and 3 miles broad is believed to have disappeared. It is evident that to carry on effectively this mechanical erosion, the sea-water must be in rapid motion. But in the deeper recesses of the ocean, where there is probably no appreciable movement of the water, there can hardly be any sensible erosion. In truth, it is only in the upper parts of the sea, which are liable to be affected by wind, that the conditions for marine erosion can be said to exist. The space within which these conditions are to be looked for is that comprised between the lowest depth to which the influence of waves and marine currents extends, and the greatest height to which breakers are thrown upon the land. These limits, no doubt, vary considerably in different regions. In some parts of the open sea, as off the coast of Florida, the disturbing action of the waves has been supposed to reach to a depth of 600 feet, though the average limit is probably greatly less. On exposed promontories in stormy seas, such as those of the north of Scot- land, breakers have been known to hurl up stones to a height of 300 feet above sea-level. But probably the zone, within which the erosive work of the sea is mainly carried on, does not as a rule exceed 300 feet in vertical range. VII MARINE EROSION 73 74 MEMORIALS LEFT BY THE SEA CHAP. Within some such limits as these, the sea is engaged in gnaw- ing away the edges of the land. A little reflection will show us that, if no counteracting operation should come into play, the pro- longed erosive action of the waves would reduce the land below the sea-level. If we suppose the average rate of demolition to be 10 feet in a century, then it would take not less than 52,800 years to cut away a strip one mile broad from the edge of the land. But while the sea is slowly eating away the coast-line, the whole surface of the land is at the same time crumbling down, and the wasted materials are being carried away by rivers into the sea at such a rate that, long before the sea could pare away more than a mere narrow selvage, the whole land might be worn down to the sea- level by air, rain, and rivers (p. 28). But there are counteracting influences in nature that would probably prevent the complete demolition of the land. What these influences are will be more fully considered in a later chapter. In the meantime, it will be enough to bear in mind that while the land is constantly worn down by the forces that are acting upon its surface, it is liable from time to time to be uplifted by other forces acting from below. And the existing relation between the amount and height of land, and the extent of sea, on the face of the globe, must be looked upon as the balance between the work- ing of both these antagonistic classes of agencies. FIG. 28. Section of submarine plain. /, Land cut into caves, tunnels, sea-stacks, reefs, and skerries by the waves, and reduced to a platform below the level of the sea (s s) on which the gravel, sand, and mud (d) produced by the waste of the coast may accumu- late. But without considering for the present whether the results of the erosion performed by the sea will be interrupted or arrested, we can readily perceive that their tendency is toward the reduc- tion of the level of the land to a submarine plain (Fig. 28). As the waves cut away slice after slice from a coast-line, the portion of land which they thus overflow, and over which they drive the shingle to and fro, is worn down until it comes below the lower limit of breaker-action, where it may be covered up with sand or vii MARINE DEPOSITS 75 mud. When the abraded land has been reduced to this level, it reaches a limit where erosion ceases, and where the sea, no longer able to wear it down further, protects it from injury by other agents of demolition. This lower limit of destruction on the sur- face of the earth has been termed " the base-level of erosion." We see, then, that the goal toward which all the wear and tear of a coast-line tends, is the formation of a more or less level plat- form cut out of the land. Yet an attentive study of the process will convince us that in the production of such a platform the sea has really had less to do than the atmospheric agents of destruc- tion. An ordinary sea-cliff is not a vertical wall. In the great majority of cases it slopes seaward at a steep angle ; but if it had been formed, and were now being cut away, mainly by the sea, it ought obviously to have receded fastest where the waves attack it that is, at its base. In other words, if sea-cliffs retired chiefly because they are demolished by the sea, they ought to be most eroded at the bottom, and should therefore be usually overhanging precipices. That this is not the case shows that some other agency is concerned which causes the higher parts of a cliff to recede faster than those below. This agency can be no other than that of the atmospheric forces air, frost, rain, and springs. These cause the face of the cliff to crumble down, detaching mass after mass, which, piled up below, serve as a breakwater, and must be broken up and removed by the waves before the solid cliff behind them can be attacked. ii. Accumulations formed by the Sea. It is not its erosive action that constitutes the most important claim of the sea to the careful study of the geologist. After all, the mere marginal belt or fringe within which this action is confined forms such a small fraction of the whole terrestrial area of the globe, that its import- ance dwindles down when we compare it with the enormously vaster surface over which the operations of the air, rain, rivers, springs, and glaciers are displayed. But when we regard the sea as the receptacle into which all the materials worn off the land ultimately find their way, we see what a large part it must play in geological history. During the last fifteen years great additions have been made to our knowledge of the sea-bottom all over the world. Portions of the deposits accumulating there have been dredged up even from the deepest abysses, so that it is now possible to construct charts, showing the general distribution of materials over the floor of the ocean. 76 MEMORIALS LEFT BY THE SEA CHAP. Beginning at the shore, let us trace the various types of marine deposits outward to the floors of the great ocean-abysses. In many places, the sea is more or less barred back by the accumu- lation of sediment worn away from the land. In estuaries, for example, there is often such an amount of mud in the water that the bottom on either side is gradually raised above the level of tide- mark, and forms eventually a series of meadows which the sea can no longer overflow. At the mouths of rivers with a consider- able current, a check is given to the flow of the water when it reaches the sea, and there is a consequent arrest of its detritus. Hence, a bar is formed across the outflow of a river, which during floods is swept seawards, and during on-shore gales is driven again inland. Even where there is no large river, the smaller streams flowing off the surface of a country may carry down sediment enough to be arrested by the sea, and to be thrown up as a long bank or bar running parallel with the coast. Behind this bar, the drainage of the interior accumulates in long lagoons, which find an outflow through some breach in the bar, or by soaking through the porous materials of the bar itself. A large part of the eastern coast of the United States is fringed with such bars and lagoons. A space several hundred miles long on the east coast of India is similarly bordered. Lut the most remarkable kind of accumulation of terrestrial detritus in the sea is undoubtedly that of river-deltas. Where the tidal scour is not too great, the sediment brought down by a large river into a marine bay or gulf gradually sinks to the bottom as the fresh spreads over and mingles with the salt water. During floods, coarse sediment is swept along, while during low states of the river nothing but fine mud may be transported. Alternating sheets of different kinds of sediment are thus laid down one upon another on the sea-floor, until by degrees they reach the surface, and thus gradually increase the breadth of the land. Some deltas are of enormous size and depth. That of the Ganges and Brah- maputra covers an area of betwee- 0,000 and 60,000 square miles that is, about as large as xgland and Wales. It has been bored through to a depth of 481 feet, and has been found to consist of numerous alternations of fine clays, marls, and sands or sandstones, with occasional layers of gravel. In all this great thickness of sediment, no trace of marine organisms was found, but land -plants and bones of terrestrial and fluviatile animals occurred. Lower Egypt has been formed by the growth of the delta of the Nile, whereby a wide tract of alluvial land has not only VII STORM-BEACHES 77 filled up the bottom of the valley, but has advanced into the Mediterranean. Turning now to the deposits that are more distinctively those of the sea itself, we find that ridges of coarse shingle, gravel, and sand are piled up along the extreme upper limit reached by the waves. The coarsest materials are for the most part thrown highest, especially in bays and narrow creeks where the breakers are confined within converging shores. In such situations, during heavy gales, storm-beaches of coarse rounded shingle are formed sometimes several yards above ordinary high-tide mark (Fig. 29). FIG. 29. Storm-beach ponding back a stream and forming a lake ; west coast of Sutherlandshire. Where a barrier of this kind is thrown across the mouth of a brook, the fresh water may be ponded back to form a small lake, of which the outflow usually escapes by percolation through the shingle. In sheltered bays, behind headlands, or on parts of a coast-line where tidal curr^*-s meet, detritus may accumulate in spits or bars. Islands hav - n this way been gradually united to each other or to the mainland, while the mainland itself has gained considerably in breadth. At Romney Marsh, on the south-east coast of England, for instance, a tract of more than 80 square miles, which in Roman times was in great part covered by the sea at high water, is now dry land, having been gained partly by the natural increase of shingle thrown up by the waves and partly by the barriers artificially erected to exclude the sea. 78 MEMORIALS LEFT BY THE SEA CHAP. While the coarsest shingle usually accumulates towards the upper part of the beach, the materials generally arrange themselves according to size and weight, becoming on the whole finer as they are traced towards low- water mark. But patches of coarse gravel may be noticed on any part of a beach, and large boulders may be seen even below the limits of the lowest tides. As a rule, the deposits formed along a beach, and in the sea immediately beyond, include the coarsest kinds of marine sediment. They are also marked by frequent alternations of coarse and fine detritus, these rapid interchanges pointing to the varying action of the waves and strong shore-currents. Towards the lower limit of breaker- action, fine gravel and sand are allowed to settle down, and beyond these, in quiet depths where the bottom is not disturbed, fine sand and mud washed away from the land slowly accumulate. The distance to which the finer detritus of the land is carried by ocean-currents before it finds its way to the bottom, varies up to about 200 miles or more. Within this belt of sea, the land- derived materials are distributed over the ocean -floor. Coarse and fine gravel and sand are the most common materials in the areas nearest the land. Beyond these lie tracts of fine sand and silt with occasional patches of gravel. Still farther from the land, at depths of 600 feet and upwards, fine blue and green muds are found, composed of minute particles of such minerals as form the ordinary rocks of the land. But traced out into the open ocean, these various deposits of recognisable terrestrial origin give place to thoroughly oceanic accumulations, especially to widespread sheets of exceedingly fine red and brown clay. This clay, the most generally diffused deposit of the deeper or abysmal parts of the sea, appears to be derived from the decomposition of volcanic fragments either washed away from volcanic islands or supplied by submarine eruptions. That it is accumulated with extreme slowness is shown by two curious and interesting kinds of evidence. Where it occurs farthest removed from land, great numbers of sharks' teeth, with ear-bones and other bones of whales, have been dredged up from it, some of these relics being quite fresh, others partially coated with a crust of brown peroxide of manganese, some wholly and thickly enveloped in this substance. The same haul of the dredge has brought up bones in all these conditions, so that they must be lying side by side on the red clay floor of the ocean abysses. The deposition of manganese is no doubt an exceedingly slow process, but it is evidently faster than the deposition of the red clay. The bones dredged up probably vii ABYSMAL DEPOSITS SUMMARY 79 represent a long succession of generations of animals. Yet so tardily does the red clay gather over them, that the older ones are not yet covered up by it, though they have had time to be deeply encased in oxide of manganese. The second kind of evidence of the extreme slowness of deposit in the ocean abysses is supplied by minute spherules of metallic iron, which occurring in numbers dispersed through the red clay, have been identified as portions of meteorites or falling stars. These particles no doubt fall all over the ocean, but it is only where the rate of deposition of sediment is exceedingly slow that they may be expected to be detected. Besides the sediments now enumerated, the bottom of the sea receives abundant accumulations of the remains of shells, corals, foraminifera and other marine creatures ; but these will be described in the next chapter, where an account is given of the various ways in which plants and animals, both upon the land and in the sea, inscribe their records in geological history. It must also be borne in mind that throughout all the sediments of the sea-floor, from the upper part of the beach down to the bottom of the deepest and remotest abyss, the remains of the plants, sponges, corals, shells, fishes and other organisms of the ocean may be entombed and preserved. It will suffice here to remember that various depths and regions of the sea have their own characteristic forms of life, the remains of which are preserved in the sediments accumulating there, and that although gravel, sand, and mud laid down beneath the sea may not differ in any recognisable detail from similar materials deposited in a lake or river, yet the presence of marine organisms in them would be enough to prove that they had been formed in the sea. It is evident, also, that if the sea- floor over a wide area were raised into land, the extent of the deposits would show that they could not have been accumulated in any mere river or lake, but must bear witness to the former presence of the sea itself. Summary. The sea records its work upon the surface of the earth in a twofold way. In the first place, in co-operation with the atmospheric agents of disintegration, it eats away the margin of the land and planes it down. The final result of this process if uninterrupted would be to reduce the level of the land to that of a submarine platform, the position of the surface of which would be determined by the lower limit of effective breaker-action. In the second place, the sea gathers over its floor all the detritus worn by every agency from the surface of the land. This material 8o MEMORIALS LEFT BY THE SEA CHAP, vn is not distributed at random ; it is assorted and arranged by the waves and currents, the coarsest portions being laid down nearest the land, and the finest in stiller and deeper water. The belt of sea-floor within which this deposition takes place probably does not much exceed a breadth of 200 miles. Beyond that belt, the bottom of the ocean is covered to a large extent with deposits of red clay derived from the decomposition of volcanic material and laid down with extreme slowness. These and the widespread deposits of dead sea-organisms (to be described in next chapter) are truly oceanic accumulations, recognisably distinct from those derived from terrestrial sources within the narrow zone of deposi- tion near the land. CHAPTER VIII HOW PLANTS AND ANIMALS INSCRIBE THEIR RECORDS IN GEOLOGICAL HISTORY BROADLY considered, there are two distinct ways in which Plants and Animals leave their mark upon the surface of the earth. In / the first place, they act directly by promoting or arresting the ' decay of the land, and by forming out of their own remains deposits which are sometimes thick and extensive. In the second place, their remains are transported and entombed in sedimentary accumulations of many different kinds, and furnish important evidence as to the conditions under which these accumulations were formed. Each of these two forms of memorial deserves our careful attention, for, taken together, they comprise the most generally interesting departments of geology, and those in which the history of the earth is principally discussed. 1 i. Direct action of living things upon the surface of the globe. This action is often of a destructive kind, both plants and animals taking their part in promoting the general disintegra- tion of rocks and soils. Thus, by their decay they furnish to the soil those organic acids referred to on pp. 16, 24, as so important in increasing the solvent power of water, and thereby promoting the waste of rocks. By thrusting their roots into crevices of cliffs, plants loosen and gradually wedge off pieces of rock, and by send- ing their roots and rootlets through the soil, they open up the subsoil to be attacked by air and descending moisture (p. 16). The action of the common earthworm in bringing up fine soil to be exposed to the influences of wind and rain was referred to at p. 1 8. Many burrowing animals also, such as the mole and 1 In the Appendix a Table of the Vegetable and Animal Kingdoms is given, from which the organic grade of the plants and animals referred to in this and subsequent chapters may be understood. G 82 RECORDS OF PLANTS AND ANIMALS CHAP. rabbit, throw up large quantities of soil and subsoil which are liable to be blown or washed away. On the other hand, the action may be conservative, as, for instance, where, by forming a covering of turf, vegetation protects the soil underneath from being rapidly removed, or where sand- loving plants bind together the surface of dunes, and thereby arrest the progress of the sand, or where forests shield a mountain-side from the effects of heavy rains and descending avalanches. (i) Deposits formed of the remains of Plants. But it is chiefly by the aggregation of their own remains into more or less extensive deposits that plants and animals leave their most prominent and enduring memorials. As examples of the way in which this is done by plants, reference may be made to peat-bogs, mangrove- swamps, infusorial earth, and calcareous sea-weeds. Peat-bogs. In temperate and arctic countries, marshy vege- tation accumulates in peat-bogs over areas from an acre or two to many square miles, and to a depth of sometimes 50 feet These deposits are largely due to the growth of bog-mosses and other aquatic plants which, dying in their lower parts, continue to grow upward on the same spot. On flat or gently-inclined moors, in hollows between hills, on valley-bottoms, and in shallow lakes, this marshy vegetation accumulates as a wet spongy fibrous mass, the lower portions of which by degrees become a more or less compact dark brown or black pulpy substance, wherein the fibrous texture, so well seen in the upper or younger parts, ' in large measure disappears. In a thick bed of peat, it is not infrequently possible to detect a suc- cession of plant remains, showing that one kind of vegetation has given place to another during **. y accumulation of the mass. In Europe, as already men- tioned on p. 4, peat -bogs often rest directly upon fresh-water marl contain- ing remains of lacustrine shells (i in Fig. 30). In every such case, it is evident that the peat has accumulated FIG. 3 o.-Section of a peat-bog. on the site of a shallow lake which has been filled up, and converted into a morass by the growth of marsh-plants along its edges and over its floor. The lowest parts of the peat may contain remains of the reeds, sedges, and other aquatic plants which choked up vin PEAT, MANGROVE-SWAMPS 83 the lake (2, 3). Higher up, the peat consists almost entirely of the matted fibres of different mosses, especially of the kind known as Bog-moss or Sphagnum (4). The uppermost layers (5, 6) may be full of roots of different heaths which spread over the surface of the bog. The rate of growth of peat has been observed in different situations in Central Europe to vary from less than a foot to about two feet in ten years ; but in more northern latitudes the growth is probably slower. Many thousand square miles of Europe and North America are covered with peat-bogs, those of Ireland being computed to occupy a seventh part of the surface of the island, or upwards of 4000 square miles. As the aquatic plants grow from the sides toward the centre of a shallow lake, they gradually cover over the surface of the water with a spongy layer of matted vegetation. Animals, and man himself, venturing on this treacherous surface sink through it, and may be drowned in the black peaty mire underneath. Long afterwards, when the morass has become firm ground, and openings are made in it for digging out the peat to be used as fuel, their bodies may be found in an excellent state of preserva- tion. The peaty water so protects them from decay that the very skin and hair sometimes remain. In Ireland, numerous skeletons of the great Irish elk have been obtained from the bogs, though the animal itself has been extinct since before the beginning of the authentic history of the country. Mangrove-swamps. Along the flat shores of tropical lands, the mangrove trees grow out into the salt water, forming a belt of jungle which runs up or completely fills the creeks and bays. So dense is the vegetation that the sand and mud; washed into the sea from the land, are arrested among the roots and radicles of the trees, and t'nu- the sea is gradually replaced by firm ground. The coast of Florida is fringed with such mangrove-swamps for a breadth of from 5 to 20 miles. In such regions, not only does the growth of these swamps add to the breadth of the land, but the sea is barred back, and prevented from attacking the newly- formed ground inside. Infusorial earth. A third kind of vegetable deposit to be referred to here is that known by the names of infusorial earth, diatom-earth, and tripoli-powder. It consists almost entirely of the minute frustules of microscopic plants called diatoms, which are found abundantly in lakes and likewise in some regions of the ocean (Fig. 31). These lowly organisms are remarkable for 8 4 RECORDS OF PLANTS AND ANIM ALS CHAP. secreting silica in their structure. As they die, their singularly durable siliceous remains fall like a fine dust on the bottom of the water, and accumulate there as a pale gray or straw-coloured deposit, which, when dry, is like flour, and in its pure varieties is made almost entirely of silica (90 to 97 per cent). Underneath the peat-bogs of Britain a layer of this material is sometimes met with. One of the most famous examples is that of Richmond, Virginia, where a bed of it occurs 30 feet thick. At Bilin in Bohemia also an important bed has long been known. The bottom of some parts of the Southern Ocean is covered with a diatom-ooze made up mainly of siliceous diatoms, but containing FIG. 31. Diatom-earth from floor of Antarctic Ocean, magnified 300 diameters (Challenger Expedition). also other siliceous organisms (radiolarians) and calcareous fora- minifera (Fig. 31). Accumulations of sea-weeds. Yet one further illustration of plant-action in the building up of solid rock may be given. As a rule the plants of the sea form no permanent accumulations, though here and there under favourable conditions, such as in bays and estuaries, they may be thrown up and buried under sand so as eventually to be compressed into a kind of peat. Some sea-weeds, however, abstract from sea-water carbonate of lime, which they secrete to such an extent as to form a hard stony structure, as in the case of the common nullipore. When the plants die, their remains are thrown ashore and pounded up by the waves, and being durable they form a white calcareous sand. By the action of the wind, this sand is blown inland and may accumulate into dunes. But unlike ordinary sand, it is liable to be slightly dissolved by rain-water, and as the portion so dis- viii NULLIPORE SAND SHELL-BANKS 85 solved is soon redeposited by the evaporation of the moisture, the little sand-grains are cemented together, and a hard crust is formed which protects the sand underneath from being blown away. Meanwhile rain-water percolating through the mounds gradually solidifies them by cementing the particles of sand to each other, and thick masses of solid white stone are thus produced. Changes of this kind have taken place on a great scale at Bermuda, where all the dry land consists of limestone formed of compacted cal- careous sand, mainly the detritus of sea-weeds. (2) Deposits formed of the remains of Animals. Animals are, on the whole, far more successful than plants in leaving enduring FIG. 32. Recent limestone (Common Cockle, etc., cemented in a matrix of broken shells). memorials of their life and work. They secrete hard outer shells and internal skeletons endowed with great durability, and capable of being piled up into thick and extensive deposits which may be solidified into compact and enduring stone. On land, we have an example of this kind of accumulation in the lacustrine marl already (pp. 4, 46) described as formed of the congregated remains of various shells. But it is in the sea that animals, secreting car- bonate of lime, build up thick masses of rock, such as shell-banks, ooze, and coral reefs (see Chapter XI, p. 158). Shell -banks. Some molluscs, such as the oyster, live in populous communities upon submarine banks. In the course of generations, thick accumulations of their shells are formed on these banks. By the action of currents also large quantities of broken shells are drifted to various parts of the sea-bottom not far from land. Such deposits of shells, in situ or transported, may be more or less mixed with or buried under sand and silt, 86 RECORDS OF PLANTS AND ANIMALS CHAP. according as the currents vary in direction and force. On the other hand, they may be gradually cemented into a solid cal- careous mass, as has been observed off the coast of Florida, where they form on the sea-bottom a sheet of limestone, made up of their remains. Ooze. From observations made during the great expedition of the Challenger, it has been estimated that in a square mile of the tropical ocean down to a depth of I oo fathoms there are more than 1 6 tons of carbonate of lime in the form of living animals. A continual rain of dead calcareous organisms is falling to the bottom, where their remains accumulate as a soft chalky ooze. FIG. 33. Globigerina ooze dredged up by Challenger Expedition from a depth of 1900 fathoms in the North Atlantic ( 2 5 5 -). Wide tracts of the ocean-floor are covered with a pale grey ooze of this nature, composed mainly of the remains of the shells of the foraminifer Globigerina (Fig. 33). In the north Atlantic this deposit probably extends not less than 1300 miles from east to west, and several hundred miles from north to south. Here and there, especially among volcanic islands, portions of the sea-bed have been raised up into land, and masses of modern limestone have thereby been exposed to view. Though they are full of the same kind of shells as are still living in the neighbouring sea, they have been cemented into compact and even somewhat crystalline rock, which has been eaten into caverns by percolating water, like limestones of much older date. This cementation, as above remarked, is due to water permeating the stone, dissolving from its outer parts the calcareous matter of shells, corallines, and other organic remains, and redepositing it again lower down, so as to cement the organic detritus into a compact stone. vin CORAL-REEFS 87 Coral-reefs offer an impressive example of how extensive masses of solid rock may be built up entirely of the aggregated remains of animals. In some of the warmer seas of the globe, and notably in the track of the great ocean-currents, where marine life is so abundant, various kinds of coral take root upon the edges and summits of submerged ridges and peaks, as well as on the shelving sea-bottom facing continents or encircling islands (i in Fig. 34). These creatures do not appear to flourish at a greater depth than 15 or 20 fathoms, and they are killed by exposure to sun and air. The vertical space within which they live may there- fore be stated broadly as about i oo feet. They grow in colonies, each composed of many individuals, but all united into one mass, which at first may be merely a little solitary clump on the sea-floor, but which, as it grows, joins other similar clumps to form what is FIG. 34. Section of a coral-reef, i. Top of the submarine ridge or bank on which the corals begin to build. 2. Coral-reef. 3. Talus of large blocks of coral-rock on which the reef is built outward. 4. Fine coral sand and mud produced by the grinding action of the breakers on the edge of the reef. 5. Coral sand thrown up by the waves and gradually accumulating above their reach to form dry ground. known as a reef. Each individual secretes from the sea-water a hard calcareous skeleton inside its transparent jelly-like body, and when it dies, this skeleton forms part of the platform upon which the next generation starts. Thus the reef is gradually built up- ward as a mass of calcareous rock (2), though only its upper sur- face is covered with living corals. These creatures continue to work upward until they reach low-water mark, and then their further upward progress is checked. But they are still able to grow outward. On the outer edges of the reef they flourish most vigorously, for there, amid the play of the breakers, they find the food that is brought to them by the ocean-currents. From time to time fragments are torn off by breakers from the reef and roll down its steep front (3). There, partly by the chemical action of the sea-water, and partly by the fine calcareous mud and sand (4), produced by the grinding action of the waves and washed into their crevices, these loose blocks are cemented into a firm, steep 88 RECORDS OF PLANTS AND ANIMALS CHAP. slope, on the top of which the reef continues to grow outwards. Blocks of coral and quantities of coral-sand are also thrown up on the surface of the reef, where by degrees they form a belt of low land above the reach of the waves (5). On the inside of the reef, where the corals cannot find the abundant food-supply afforded by the open water outside, they dwindle and die. Thus the tendency of all reefs must be to grow seawards, and to increase in breadth. Perhaps their breadth may afford some indication of their rela- tive age. Where a reef has started on a shelving sea-bottom near the coast of a continent, or round a volcanic island, the space of water inside is termed the Lagoon Channel. Where the reef has been built up on some submarine ridge or peak, and there is con- sequently no land inside, the enclosed space of water is called a Lagoon^ and the circular reef of coral is known as an Atoll. If no subsidence of the sea-bottom takes place, the maximum thickness of a reef must be limited by the space within which the corals can thrive that is, a vertical depth of about I oo feet from the surface of the sea. But the effect of the destruction of the ocean-front of the reef, and the piling up of a slope of its fragments on the sea- bottom outside, will be to furnish a platform of the same materials on which the reef itself may grow outward, so that the united mass of calcareous rock may attain a very much greater thickness than 100 feet. On the other hand, if the sea-bottom were to sink at so slow a rate that the reef-building corals could keep pace with the subsidence, a mass of calcareous rock many thousand feet thick might obviously be formed by them. It is a disputed ques- tion in which of these two ways atolls have been formed. It is remarkable how rapidly and completely the structure of the coral-skeleton is effaced from the coral-rock, and a more or less crystalline and compact texture is put in its place. The change is brought about partly by the action of both sea-water and rain-water in dissolving and redepositing carbonate of lime among the minute interstices of the rock, and partly also by the abundant mud and sand produced by the pounding action of the breakers on the reef, and washed into the crevices. On the portion of a reef laid dry at low water, the coral-rock looks in many places as solid and old as some of the ancient white lime- stones and marbles of the land. There, in pools where a current or ripple of water keeps the grains of coral-sand in motion, each grain may be seen to have taken a spherical form unlike that of the ordinary irregularly rounded or angular particles. This arises vin ENTOMBMENT OF PLANTS AND ANIMALS 89 because carbonate of lime in solution in the water is deposited round each grain as it moves along. A mass of such grains aggregated together is called oolite, from its resemblance to fish- roe. In many limestones, now forming wide tracts of richly culti- vated country, this oolitic structure is strikingly exhibited. There can be no doubt that in these cases it was produced in a similar way to that now in progress on coral-reefs (see pp. 141, 155). In the coral tracts of the Pacific Ocean there are nearly 300 coral islands, besides extensive reefs round volcanic islands. Others occur in the Indian Ocean. Coral-reefs abound in the West Indian Seas, where, on many of the islands, they have been upraised into dry land, in Cuba to a height of 1 1 oo feet above sea-level. The Great Barrier Reef that fronts the north-eastern coast of Australia is 1250 miles long, and from 10 to 90 miles broad. There are other ways in which the aggregation of animal remains forms more or less extensive and durable rocks. To some of these reference will be made in later chapters. Enough has been said here to show that by the accumulation of their hard parts animals leave permanent records of their presence both on land and in the sea. ii. Preservation of remains of Plants and Animals in sedimentary deposits. But it is not only in rocks formed out of their remains that living things leave their enduring records. These remains may be preserved in almost every kind of deposit, under the most wonderful variety of conditions. And as it is in large measure from their occurrence in such deposits that the geologist derives the evidence that successive tribes of plants and animals have peopled the globe, and that the climate and geo- graphy of the earth have greatly varied at different periods, we shall find it useful to observe the different ways in which the remains both of plants and animals are at this moment being entombed and preserved upon the land and in the sea. With the knowledge thus gained, it will be easier to understand the lessons taught by the organic remains that lie among the various solid rocks around us. It is evident that in the vast majority of cases, the plants and animals of the land leave no perceptible trace of their presence. Of the forests that once covered so much of Central and Northern Europe, which is now bare ground, most have disappeared, and unless authentic history told that they had once flourished, we should never have known anything about them. There were also 90 RECORDS OF PLANTS AND ANIMALS CHAP. herds of wild oxen, bears, wolves, and other denizens contempor- aneous with the vanished forests. But they too have passed away, and we might ransack the soil in vain for any trace of them. If the remains of terrestrial vegetation and animals are any- where preserved it must obviously be only locally, but the favour- able circumstances for their preservation, although not everywhere to be found, do present themselves in many places if we seek for them. The fundamental condition is that the relics should, as soon as possible after death, be so covered up as to be protected from the air and from too rapid decomposition. Where this con- dition is fulfilled, the more durable of them may be preserved for an indefinite series of ages. (a) On the Land there are various places where the remains both of plants and animals are buried and shielded from decay. To some of these reference has already been made. Thus amid the fine silt, mud, and marl gathering on the floors of lakes, leaves, fruits, and branches, or tree -trunks, washed from the neighbouring shores, may be imbedded, together with insects, birds, fishes, lizards, frogs, field-mice, rabbits, and other inhabitants. These remains may of course often decay on the lake-bottom, but where they sink into or are quickly covered up by the sedi- ment, they may be effectually preserved from obliteration. They undergo a change, indeed, being gradually turned into stone, as will be described in Chapter XV. But this conversion may be effected so gently as to retain the finest microscopic textures of the original organisms. In peat-bogs also, as already stated (p. 83), wild animals are often engulfed, and their soft parts are occasionally preserved as well as their skeletons. The deltas of river-mouths must receive abundantly the remains of animals swept off by floods. As the carcases float seawards, they begin to fall to pieces and the separate bones sink to the bottom, where they are soon buried in the silt. Among the first bones to separate from the rest of the skeleton are the lower jaws (pp. 308, 311). We should therefore expect that in excavations made in a delta these bones would occur most frequently. The rest of the skeleton is apt to be carried farther out to sea before it can find its way to the bottom. The stalagmite floor of caverns has already been referred to (p. 56) as an admirable material for enclosing and preserving organic remains. The animals that fell into these recesses, or used them as dens in which they lived or into which they dragged their prey, have left their bones on the floors, where, viii ORGANIC REMAINS ON THE SEA-FLOOR 91 encased in or covered by solid stalagmite, these relics have remained for ages. Most of our knowledge of the animals which inhabited Europe at the time when man appeared, is derived from the materials -disinterred from these Bone-caves. Allusion has also been made to the travertine formed by mineral-springs and to the facility with which leaves, shells, insects, and small birds, reptiles, or mammals maybe enclosed and preserved in it (p. 58). Thus, while the plants and animals of the land for the most part die and decay into mere mould, there are here and there localities where their remains are covered up from decay and preserved as memorials of the life of the time. (ft) On the bottom of the Sea the conditions for the pre- servation of organic remains are more general and favourable than on land: Among the sands and gravels of the shore, some of the stronger shells that live in the shallower waters near land may be covered up and preserved, though often only in rolled fragments. It is below tide-mark, however, and more especially beneath the limit to which the disturbing action of breakers descends, that the remains of the denizens of the sea are most likely to be buried in sediment and to be preserved there as memorials of the life of the sea. It is evident that hard and therefore durable relics have the best chance of escaping destruction. Shells, corals, corallines, spicules of sponges, teeth, vertebras, and ear-bones of fishes may be securely entombed in successive layers of silt or mud. But the vast crowds of marine creatures that have no hard parts must almost always perish without leaving any trace whatever of their existence. And even in the case of those which possess hard shells or skeletons, it will be easily understood that the great majority of them must be decomposed upon the sea-bottom, their component elements passing back again into the sea-water from which they were originally derived. It is only where sediment is deposited fast enough to cover them up and protect them before they have time to decay, that they may be expected to be preserved. In the most favourable circumstances, therefore, only a very small proportion of the creatures living in the sea at any time leave a tangible record of their presence in the deposits of the sea- bottom. It is in the upper waters of the ocean, and especially in the neighbourhood of land, that life is most abundant. The same region also is that in which the sediment derived from the waste of the land is chiefly distributed. Hence it is in these marginal parts of the ocean that the conditions for preserving memorials of the animals that inhabit the sea are best developed. 92 RECORDS OF PLANTS AND ANIMALS CHAP, vm As we recede from the land, the rate of deposit of sediment on the sea-floor gradually diminishes, until in the central abysses it reaches that feeble stage so strikingly brought before us by the evidence of the manganese nodules (p. 78). The larger and thinner calcareous organisms are attacked by the sea-water and dissolved, apparently before they can sink to the bottom ; at least their remains are comparatively rarely found there. It is such indestructible objects as sharks' teeth and vertebrae and ear-bones of whales that form the most conspicuous organic relics in these abysmal deposits. Summary. Plants and animals leave their records in geo- logical history, partly by forming distinct accumulations of their remains, partly by contributing their remains to be imbedded in different kinds of deposits both on land and in the sea. As examples of the first mode of chronicling their existence, we may take the growth of marsh-plants in peat-bogs, the spread of mangrove -swamps along tropical shores, and the deposition of infusorial earth on the bottom of lakes and of the sea ; the accumulation of nullipore sand into solid stone, the formation of extensive shell-banks in many seas, the wide diffusion of organic ooze over the floor of the sea, and the growth of coral reefs. As illustrations of the second method, we may cite the manner in which the remains of terrestrial plants and animals are preserved in peat-bogs, in the deltas of rivers, in the stalagmite of caverns, and in the travertine of springs ; and the way in which the hard parts of marine creatures are entombed in the sediments of the sea-floor, more especially along that belt fringing the continents and islands, where the chief deposit of sediment from the disinte- gration of the land takes place. Nevertheless, alike on land and sea, the proportion of organic remains thus sealed up and pre- served is probably always but an insignificant part of the total population of plants and animals living at any given moment. How the remains of plants and animals when once entombed in sediment are then hardened and petrified, so as to retain their minute structures, and to be capable of enduring for untold ages, will be treated of in Chapter XV. CHAPTER IX THE RECORDS LEFT BY VOLCANOES AND EARTHQUAKES THE geological changes described in the foregoing chapters affect only the surface of the earth. A little reflection will convince us that they may all be referred to one common source of energy the sun. It is chiefly to the daily influence of that great centre of heat and light that we must ascribe the ceaseless movements of the atmosphere, the phenomena of evaporation and condensation, the circulation of water over the land, the waves and currents of the sea, in short the whole complex system which constitutes what has been called the Life of the Earth. Could this influence be conceivably withdrawn, the planet would become cold, dark, silent, lifeless. But besides the continual transformations of its surface due to solar energy, our globe possesses distinct energy of its own. Its movements of rotation and revolution, for example, provide a vast store of force, whereby many of the most important geological processes are initiated or modified, as in the phenomena of day and night and the seasons, with the innumerable meteorological and other effects that flow therefrom. These movements, though slowly growing feebler, bear witness to the wonderful vigour of the earlier phases of the earth's existence. Inside the globe too lies a vast magazine of planetary energy in the form of an interior of intensely hot material.' The cool outer shell is but an insigni- ficant part of the total bulk of the globe. To this cool part the name of " crust " was given at a time when the earth was believed to consist of an inner molten nucleus enclosed within an outer solid shell or crust. The term is now used merely to denote the cool solid external part of the globe, without implying any theory as to the nature of the interior. Condition of the Earth's Interior. It is obvious that we are 94 VOLCANOES AND EARTHQUAKES CHAP. not likely ever to learn by direct observation what may be the condition of the interior of our planet. The cool solid outer shell is far too thick to be pierced through by human efforts ; but by various kinds of observations, more or less probable conclusions may be drawn with regard to this problem. In the first place, it has been ascertained that all over the world, wherever borings are made for water or in mining operations, the temperature increases in proportion to the depth pierced, and that the average rate of increase amounts to about one degree Fahrenheit for every 64 feet of descent. If the rise of temperature continues inward at this rate, or at any rate at all approaching it, then at a distance from the surface, which in proportion to the bulk of the whole globe is comparatively trifling, the heat must be as great as that at which the ordinary materials of the crust would melt at the surface. In the second place, thermal springs in all quarters of the globe, rising sometimes with the temperature of boiling water, and occa- sionally even still hotter, prove that the interior of the planet must be very much warmer than its exterior. In the third place, volcanoes widely distributed over the earth's surface throw out steam and heated vapours, red-hot stones, and streams of molten rock. It is quite certain therefore that the interior of the globe must be intensely hot ; but whether it is actually molten or solid has been the subject of prolonged discussion. Three opinions have found stout defenders, (i) The older geologists maintained that the phenomena of volcanoes and earthquakes could not be explained, except on the supposition of a crust only a few miles thick, enclosing a vast central ocean of molten material. (2) This view has been opposed by physicists who have shown that the globe, if this were actually its structure, could not resist the attraction of sun and moon, but would be drawn out of shape, as the ocean is in the phenomenon of the tides, and that the absence of any appreciable tidal deformation in the crust shows that the earth must be practically solid and as rigid as a ball of glass, or of steel. (3) A third opinion has been advanced by geologists who, while admitting that the earth behaves on the whole as a solid rigid body, yet believe that many geological phenomena can only be explained by the existence of some liquid mass hgneath the crust. Accordingly they suppose that while the nucleus is retained in the solid state by the enormous superincumbent pressure -under which it lies, and the crust has become solid by cooling, there is an intermediate liquid or viscous layer which has ix NATURE OF VOLCANOES 95 not yet cooled sufficiently to pass into the solid crust above, and does not lie under sufficient pressure to form part of the solid nucleus below. At present, the balance of evidence and argument seems to be in favour of the practical rigidity and solidity of the globe as a whole. But the materials of its interior must possess temperatures far higher than those at which they would melt at the surface. They are no doubt kept solid by the vast overlying pressure, and any change which could relieve them of this pressure would allow them to pass into the liquid form. This subject will be again alluded to in Chapter XVI. Meanwhile, let us consider how the intensely hot nucleus of the planet reacts upon its surface. Rocks are bad conductors of heat. So slowly is the heat of the interior conducted upwards by them that the temperature of the surface of the crust is not appreciably affected by that of the intensely hot nucleus. But the fact that the surface is not warmed from this source shows that the heat of the interior must pass off into space as fast as it arrives at the surface, and proves that our planet is gradually cooling. For many millions of years the earth has been radiating heat into space, and has consequently been losing energy. Its present store of planetary vitality therefore must be regarded as greatly less than it once was. VOLCANOES. Of all the manifestations of this planetary vitality, by far the most impressive are those furnished by volcanoes. The general characters of these vents of communication between the hot interior and cool surface of the planet are doubtless already familiar to the reader of these chapters the volcano itself, a conical hill or mountain, formed mainly or entirely of materials ejected from below, having on its truncated summit the basin-shaped crater^ at the bottom of which lies the vent or funnel from which, as well as from rents on the flanks of the cone, hot vapours, cinders, ashes, and streams of molten lava are discharged, till they gradually pile up the volcanic cone round the vent whence they escape. A volcanic cone, so long as it remains, bears eloquent testimony to the nature of the causes that produced it. Even many centuries after it has ceased to be active, when no vapours rise from any part of its cold, silent, an'd motionless surface, its conical form, its cup-shaped crater, its slopes of loose ashes, and its black bristling lava-currents remain as unimpeachable witnesses that the volcanic 96 VOLCANOES AND EARTHQUAKES CHAP. fires, now quenched, once blazed forth fiercely. The wonderful groups of volcanoes in Auvergne and the Eifel are as fresh as if they had not yet ceased to be active, and might break forth again at any moment ; yet they have been quiescent ever since the beginning of authentic human history. But in the progress of the degradation which everywhere slowly changes the face of the land, it is impossible that volcanic hills should escape the waste which befalls every other kind of eminence. We can picture a time when the volcanic cones of Auvergne will have been worn away, and when the lava-streams that descend from them will be cut into ravines and isolated into separate masses by the streams that have even already deeply trenched them. Where all the ordinary and familiar signs of a volcano have been removed, how can we tell that any volcano ever existed ? "What enduring record do volcanoes inscribe in geo- logical history ? Now, it must be obvious that among the operations of an active volcano, many of the most striking phenomena have hardly any importance as aids in recognising the traces of long -extinct vol- canic action. The earthquakes and tremors that accompany volcanic outbursts, the constant and prodigious out-rushing of steam, the abundant discharge of gases and acid vapours, though singularly impressive at the time, leave little or no lasting mark of their occurrence. It is not in phenomena, so to speak, transient in their effects, that we must seek for a guide in exploring the records of ancient volcanoes, but in those which fracture or other- wise affect the rocks below ground, and pile up heaps of material above. Keeping this aim before us, we may obtain from an examina- tion of what takes place at an active volcano such durable proofs of volcanic energy as will enable us to recognise the former exist- ence of volcanoes over many tracts of the globe where human eye has never witnessed an eruption, and where, indeed, all trace of what could be called a volcano has utterly vanished. A method of observation and reasoning has been established, from the use of which we learn that in some countries, Britain for example, though there is now no sign of volcanic activity, there has been a succes- sion of volcanoes during many protracted and widely separated periods, and that probably the interval that has passed away since the last eruptions is not so vast as that which separated these from those that preceded them. A similar story has been made out in many parts of the continent of Europe, in the United States, India, ix VOLCANIC PRODUCTS 97 and New Zealand, and, indeed, in most countries where the subject has been fully investigated. A little reflection on this question will convince us that the permanent records of volcanic action must be of two kinds : first and most obvious are the piles of volcanic materials which have been spread out upon the surface of the earth, not only round the immediate vents of eruption, but often to great distances from them ; secondly, the rents and other openings in the solid crust of the earth caused by the volcanic explosions, and some of which have served as channels by which the volcanic materials have been expelled to the surface. Volcanic Products. We shall first consider those materials which are erupted from volcanic vents and are heaped up on the surface as volcanic cones or spread out as sheets. They may be conveniently divided into two groups: ist, Lava, and 2d, Frag- mentary materials.- (i) Lava. Under this name are comprised all the molten rocks of volcanoes. These rocks present many varieties in composition FIG. 35. Cellular Lava with a few of the cells filled up with infiltrated mineral matter (Amygdules). and texture, some of the more important of which will be described in Chapter XI. Most of them are crystalline that is, are made up wholly or in greater part of crystals of two or more minerals inter- locked and felted together into a coherent mass. Some are chiefly composed of a dark brown or black glass, while others consist of a compact stony substance with abundant crystals imbedded in it. Probably most of them when in completest fusion within the earth's crust existed in the condition of thoroughly molten glass, H VOLCANOES AND EARTHQUAKES CHAP. the transition from that state to a stony or lithoid one being due to a process of "devitrification" (p. 144) consequent on cooling. During this process some of the component ingredients of the glass crystallise out as separate minerals, and this crystallisation some- times proceeds so far as to use up all the glass and to transform it into a completely crystalline substance. In many cases lavas are strikingly cellular that is to say, they contain a large number of spherical or almond-shaped cavities somewhat like those of a sponge or of bread, formed by the expan- sion of the steam absorbed in the molten rock (Figs. 35 and 36 and p. 146). Lavas vary much in weight and in colour. The heavier kinds are more than three times the weight of water ; or, in other words, they have a specific gravity ranging up to 3.3 ; and are commonly dark grey to black. The lighter varieties, on the other hand, are little more than twice the weight of water, or have a specific gravity which may be as low as 2.3, while their colours are usually paler, sometimes almost white. When lava is poured out at the surface it issues at a white heat that is, at a temperature sometimes above that of melting copper, or more than 2204 Fahr.; but its surface rapidly darkens, cools, and hardens into a solid crust which varies in aspect according to the liquidity of the mass. Some lavas are remarkably fluid, flow- ing along swiftly like melted iron ; others move sluggishly in a stiff viscous stream. In many pasty lavas, the surface breaks up into rough cindery blocks or scoriae like the slags of a foundry, which grind upon each other as the still molten stream underneath creeps forward (p. 146). In general, the upper part of a lava-stream is more cellular than the central portions, no doubt because the imprisoned steam can there more easily expand. The bottom, too, is often rough and slaggy, as the lava is cooled by contact with the ground, and por- tions of the chilled bottom- crust are pushed along or broken up and involved in the still fluid portion above. There are thus three more or less well-defined zones in a solidi- fied lava-current a cellular or slaggy upper part (c in Fig. 36), FIG. 36. Section of a lava-current. ix LAVA-STREAMS 99 a more solid and jointed centre (b\ embracing usually by much the largest proportion of the whole, and a cellular or slaggy bottom (a). A rock presenting these characters tells its story of volcanic action in quite unmistakable language. It remains as evidence of the existence of some neighbouring volcanic vent, now perhaps entirely covered up, whence it flowed. We may even be able to detect the direction in which the lava moved. The cells opened by the segregation and expansion of the steam entangled in the interstices of a mass of lava which is at rest are, on the whole, spherical. But if the rock is still moving, the cells will be drawn out and flattened into almond-shaped (amygdaloidal) vesicles, with their flat 'sides parallel to the surface of the lava, and their longer *FiG. 37. Elongation of cells in direction of flow of a lava-stream. axes ranged in one general direction, which is that of the motion of the molten stream (Fig. 37). At a volcanic vent, the mass of erupted lava is generally thickest, and it thins away as its successive streams terminate on the lower grounds surrounding the cone. But sometimes a lava- current may flow for 40 miles or more from its source, and may here and there attain locally a great thickness by rolling into a valley and filling it up, as has been witnessed among the Icelandic eruptions. As a rule, where ancient lava-streams are found to thicken in a certain direction, we may reasonably infer that in that direction lay the vent from which they flowed. Again, sheets of lava that solidify on the slopes of a volcanic cone are inclined ; they may congeal on declivities of as much as 30 or 40. If a series of ancient lavas were observed to slope upward to a common centre, we might search there for some trace ioo VOLCANOES AND EARTHQUAKES CHAP. of the funnel from which they were discharged. But, of course, in proportion to their antiquity, lava-streams, like every other kind of rock, have suffered from geological revolutions, among which those that involve upheaval and dislocation are especially important, so that the inclination of an ancient lava-bed must not be too hastily assumed as an indication of the slope of the cone of a vol- cano. It must be taken in connection with the rest of the evidence supplied by the whole district. Where lavas reach the lower grounds beyond the foot of a volcanic cone, they may spread out in wide nearly horizontal sheets. As current succeeds current, the original features of the plain may be entirely buried under a mass of lava many feet thick. If a section could be cut through such an accumulation, it might be possible to determine the thickness of each successive lava- stream by means of the slaggy upper and lower surfaces. Here and there, too, where two eruptions were separated by an interval long enough to allow the surface of the older mass partially to crumble into soil and support some vegetation, the layer "of burnt soil between the two sheets of lava would remain as a witness of this interval. In other instances, we can understand that in the larger hollows of a lava-plain, ponds or lakes might gather, on the floor of which there might be deposited layers of fine silt full of terrestrial leaves, insect remains, land and fresh -water shells, and other organic relics of a land-surface. If, now, a lacustrine accumulation of this kind were to be buried under a new outburst of lava, it would be sealed up and might preserve its record intact for vast ages. In any section cut through such a series of lava-beds by a river or the sea, or by man, the layers of silt with their organic remains intercalated between the lava-streams would prove the eruptions to have taken place on land, and to have been separated by a long interval, during which a lake was formed on the cold and decom- posing surface of the earlier lava. The conditions under which the volcanic outbursts occurred may be thus inferred, not so much from the nature of the volcanic materials themselves, as from that of the layers of sediment that may happen to have been preserved among them. Seams of red baked soil, with charred remains of terrestrial vegetation interposed between the upper and under sides of successive lavas would point to subaerial eruptions. Bands of hardened clay or marl, with leaves and fresh-water shells, would show that the lavas had in- vaded a lake. Beds of limestone or other rock, containing corals, ix VOLCANIC TUFFS 101 sponges, marine shells, and other traces of the life of the sea, would demonstrate that the eruptions were submarine. Examples of each of these varieties of evidence occur abundantly among the old volcanic tracts of Britain. (2) Fragmentary Products. These supply some of the most striking proofs of volcanic energy. They vary in size from huge blocks of stone, weighing many tons, down to the finest dust. The coarsest materials naturally accumulate round the vent, while the finest may be borne away by wind to distances of many hundreds of miles. On the volcano itself, the stones, ashes, and dust form beds of coarse and fine texture which, on the outside of the cone, have the usual slope of the declivity. By degrees, they become more or less consolidated, and are then known by the general name of Tuffs. (See p. 153.) The materials composing a tuff are generally derived from lavas. The fine dust, discharged from a large volcano in such prodigious quantities as to make the sky dark as midnight for days to- gether, is simply lava that has been blown into this finely divided condition by the explosion of the vapours and gases which exist absorbed in it while still deep down within the earth's crust. The cinder-like fragments, and scoriae or slags, that are ejected in such numbers and fall back into the crater and upon the outer slopes of the cone, are pieces of lava frothed up by the expansion of the imprisoned steam, torn off from the column of lava in the vent, and shot whirling up into the air. Large blocks of lava, or of the rocks through which the volcanic funnel has been opened, are often broken off by the force of the explosions and discharged with the other volcanic detritus from the vent. These materials, descending to the ground, form successive beds that vary in dimensions according to the vigour of eruption and their distance from the vent. Around the focus of activity there may be thick accumulations of blocks, bombs, and pieces of scoriae mixed with fine ashes and sand. A notable feature is the generally cellular character of these stones a peculiarity which marks them as made of truly volcanic materials. An examination of the finest dust likewise discloses the presence of the glass and crystals that constitute the lava from the explosion of which the dust was derived (p. 143). From the wide area over which the fragmentary materials ejected by volcanoes are dispersed in the atmosphere before they fall to the earth, they are more likely than lavas, to be preserved among contemporaneous sedimentary accumulations. They often 102 VOLCANOES AND EARTHQUAKES CHAP. descend upon lakes, and must there be interstratified with the mud, marl, or other deposit in progress at the time. They are also widely diffused over the sea-floor. Recent dredgings of the ocean-basins have shown that traces of fine volcanic detritus may be detected even at remote distances from land. As active volcanoes almost always rise near the sea, as the oceans are dotted over with volcanic islands, and as, doubtless, many eruptions take place on the sea-bottom, there are obvious reasons why volcanic particles should be universally diffused over the sea-bottom. Geolo- gists can also understand why, in the records of volcanic history in bygone ages, so large a proportion of the evidence should be of submarine eruptions. Beds of tuff often contain traces of the plants or animals that lived on the surfaces on which the volcanic materials fell some- times remains of terrestrial or lacustrine vegetation and animals ; but in the great majority of instances, shells and other relics of the inhabitants of the sea. In a series of layers of tuff round a volcanic orifice, the memor- ials of the earliest discharges are of course preserved in the layers at the bottom. Accordingly, in such situations, abundant frag- ments of the rocks of the surrounding countiy may be noticed. We could hardly ask for more convincing evidence of the blowing out of the vent and the ejection of the rock-fragments from it, before the volcano began to discharge only volcanic materials. Large blocks of lava ejected obliquely from the crater may fall beyond the limits of the cone. If a block thus discharged should fall into a lake-basin, it would be covered up in the silt ac- cumulating there, and might be the only remaining record of the eruption to which it belonged. In after times, were the lake-floor laid -dry, the stone might be found in its place, the layers of sedi- FIG. 38. Volcanic block ejected during the deposition of strata in water. ment into which it fell pressed down by the force with which it landed on them and by its weight, the later layers mounting over and covering it. Examples of this kind of evidence may be gathered in many old volcanic districts. One taken from the coast of Fife is given in Fig. 38. The lowest bed there shown (i) is a brown shaly fire-clay, about 5 inches thick, which was once a vegetable soil, for abundant rootlets can be seen branching ix VENTS AND FISSURES 103 through it. It is overlain by a seam of coal (2), 5 or 6 inches thick, representing the dense growth of vegetation that flourished upon the soil ; the next layer (3) is a green crumbling fire-clay about a foot thick, covered by a dark shale with remains of plants (4). The feature of special interest in this section is the angular block of lava (diabase) weighing about six or eight pounds, which is stuck vertically in bed No. 3. There can be no doubt that this was ejected by an explosion, of which there is here no other record. It probably descended with some force, for, as shown in the drawing, the lower layers of the fire-clay are pressed down by it, and the coal itself is compressed. We see that the stone fell before the upper half of the fire-clay was formed, for the layers of that part of the bed are heaped around the stone and finally spread over it. There are layers of lava and tuff above and below the strata depicted in this section, so that there is abundant other evidence of preceding and subsequent volcanic action here (see Fig. 109). As the fine volcanic dust may be transported by wind for hundreds of miles before reaching the surface of the earth, its presence does not necessarily show that a volcano existed in the neighbourhood in which it fell. The fine ashes from the Icelandic volcanoes, for example, have been found abundantly even as far as Sweden and the Orkney Islands. But where the fragmentary materials are of coarse grain, and more especially where they contain large slags, scoriae, and bombs, and where they are inter- stratified with sheets of lava, they unquestionably indicate the proximity of some volcanic vent from which the whole proceeded. Volcanic Vents and Fissures. The various materials ejected from a volcano to the surface may conceivably be in course of time entirely swept away. Nevertheless, though every sheet of lava and every bed of tuff is removed, there will still remain the filled-up vent) funnel, or fissure, up which these materials rose, and out of which they were ejected. This opening in the solid crust of the earth must evidently be one of the most durable, as it is certainly one of the fundamental features in a volcano. Let us first consider the vent of an ordinary volcano. In many instances, there is reason to believe that volcanic vents are opened along lines of fracture in the earth's crust. This seems especially to be the case where a group of volcanoes runs along one definite line, as is represented in Fig. 39. Along such a fissure, either of older date or due to the energy of the volcanic explosions themselves, there must be weaker places where the 104 VOLCANOES AND EARTHQUAKES CHAP. overlying mass is less able to bear the strain of the pent-up vapours underneath. At these places, after successive shocks, openings may at length be made to the surface, whence the lavas and ashes will be emitted, and each such opening will be marked by a cone of the erupted material. But in innumerable examples, it is found that a fissure is not necessary for the formation of a volcano. The effect of the volcanic explosions is such as to drill a pipe or funnel even through the solid unfractured crust of the earth. The volcanic energy, so far from requiring a line of fracture for its assistance, seems often to have avoided making use of such a line, even when it existed. In the volcanic plateaux of Utah, which are dotted over with little volcanic cones, and are also traversed by great FIG. 39. Volcanoes on lines of fissure. dislocations, it is noticeable that the vents are not clustered along the lines of fracture. In the same region, and also along the courses of the Rhine and Moselle, volcanic vents have been opened near the brink of deep ravines rather than at the bottom. These are features of volcanic action, for which no satisfactory explanation has yet been found. The crater of an active volcano is a hideous yawning cauldron, with rough red and black walls, at the bottom of which lie steam- ing pools of molten lava. Every now and then a sharp explosion tears the lava open, sending up a shower of glowing fragments and hot ashes. These pools of liquid lava lie evidently on the top of a column of melted rock which descends in the volcanic chimney to an unknown depth into the earth's interior. If the volcano were to become extinct, this lava column would cool and solidify, and even after the entire destruction and removal of the cone and crater, would remain as a stump to tell where the site of the volcano had been. Layer after layer might be stripped off ix STRUCTURE OF VOLCANIC VENTS 105 the surface of the land ; hundreds or thousands of feet of rock might in this manner be removed, yet, so far as we know, the stump of the volcano would still be there. No probable amount of waste of the surface of the earth's crust could remove a vertical column of rock which descends to an unknown depth into the interior. ; The site of a volcanic vent can never be effaced except by being buried under masses of younger rock. Volcanic vents, affording as they do so durable a testimony to volcanic action, deserve careful attention. At an active volcano, or even at one which, though extinct, still retains its cone of erupted materials, we cannot, of course, learn much regarding the shape and size of the funnel, for only the crater, and at most merely the upper part of the vent, are accessible. But among volcanic tracts of older date, where the cones have been destroyed, and where the filled-up funnels are laid bare, the subterranean architecture of volcanoes is revealed to us. At such places we are allowed, as it were, to descend the chimney of a volcano, and to make observations altogether impossible at a modern volcanic cone. From observations made at such favourable localities, it has been ascertained that the funnels of volcanoes are in general rudely circular or elliptical, though liable to many modifications of outline. They vary indefinitely in diameter, according to the vigour of the volcanic outbursts that produced them. The smaller vents are not more than a few yards in width; but those of the larger volcanoes which, as in Sumatra and Java, have sometimes craters comprising an area of 40 square miles, must have enormously larger funnels. The materials that fill up a vent are sometimes only fragments of the surrounding rocks. In such cases, we may suppose that when the volcanic explosions had spent their force and had blown out an opening to the surface of the ground, they were not succeeded by the uprise of any solid volcanic materials ; that, in short, only the first stage in the establishment of a volcano was reached, when, owing to some failure of the subterranean energy at the place, the operations came to an end. But though the upper part of the vent might remain open, surrounded with a crater formed of the fragments into which the rocks were blown by the explosions, the lower parts would undoubtedly be filled up by the fall of fragments back again into the vent. And if all the material ejected to the surface were removed, the top of this column of fragmentary materials would remain as an unmistakable evidence of the explosions that had originated, it. io6 VOLCANOES AND EARTHQUAKES CHAP. But in the vast majority of cases, the operations at a volcanic vent do not end with the first explosions. Clouds of ashes and stones are ejected, and streams of molten lava are poured forth. In some instances, the chimney may be finally choked with vol- canic blocks, scoriae, cinders, and ashes, in others with consolidated lava. Examples of both kinds of infilling are found, and also others where the two forms of volcanic material occur together in the same vent. A volcanic chimney filled up in this way with volcanic materials, and exposed by the removal of the lava or ashes thrown out to the surface is known as a Neck (see Figs. 40, 41, 42, 1 1 1, and p. 208). As these materials are usually harder and more durable than the sur- rounding rocks, they project above the general surface of the ground. The stump of the volcano is left as a hill, the form and prominence of which will chiefly depend upon the nature of the material ; hard tough lava will rise abruptly, as a crag or hill, above the sur- rounding country, while consolidated ashes, scoriae, and other fragmentary stuff will give a smoother and less marked outline. These features will be best understood from a series of diagrams. FIG. 40. Outline of a Volcanic Neck. We may take, by way of illustration, a neck composed mainly of fragmentary ejections, but with a plug of lava reaching its summit. The usual outlines of such a neck are represented in Fig. 40. There is nothing in the general form of this hill to suggest a volcanic origin ; yet, if we examine its structure and that of the ground around it, we may find them to be as re- presented in Fig. 41, where the surrou nding rocks are supposed FIG. 4i.-Ground-plan of the structure of the Neck shown in Fig. 40. Qf stones, clays, limestones, and other sedimentary deposits (#), through which the volcanic vent (b, c) has been drilled. The ix VOLCANIC NECKS 107 neck is represented as elliptical in cross section, composed mainly of consolidated volcanic ashes and blocks (b\ but with a mass of lava (c} in the centre. The structure of the hill is explained in the vertical section, Fig. 42 (see also Fig. in). We there see that the vent has been blown through the surrounding strata (a, a), and has been filled up mainly with fragmentary materials (b, b) ; but that through its centre there has risen a column or plug of lava (c\ which not improbably marks the last effort of the volcano to force solid ejections to the surface. The line J, J indi- cates the present surface of the ground, after the prolonged waste during which all the volcanic cone has been removed. But we a * a FIG. 42. Section through the same Neck as in Figs. 40 and 41. can in imagination restore the original surface, which may have been somewhat as shown by the dotted lines, the position of the crater being indicated at e, and its crest on either side at d, d. No trace is here left of the original volcanic cone. The present form of the ground is due to denudation, which has left the more durable volcanic rocks projecting above the surrounding strata. The continued progress of superficial degradation will remove still more of the neck, but the downward continuation of the volcanic column must always remain, and will probably always project as a hill. A volcanic neck is thus one of the most enduring and unmistakable evidences of the site of a volcano (see p. 208). Besides vents or funnels, other openings are made by volcanic explosions in the crust, which serve as receptacles of lava and ashes, and remain as durable memorials of volcanic action. Of these the most important are Fissures, which are formed in large numbers in and around a volcanic cone, but which may also arise at a distance from any actual volcano. During the convulsions of an eruption, the cone and the surrounding country are sometimes split by lines of fissure, which tend to io8 VOLCANOES AND EARTHQUAKES CHAP. radiate from the centre of disturbance, somewhat as cracks do in a pane of glass through which a stone is thrown. Sometimes the two sides of a fissure close together again, leaving no superficial trace of the dislocation. More frequently steam and various volcanic vapours escape from the chasm, and may deposit along the walls sublimates of different minerals, such as common salt, chloride of iron, specular iron, sulphur, and sal-ammoniac. These deposited substances may even continue to grow there until they entirely fill up the space between. In such cases, the line of fissure is marked by a vertical or steeply inclined band of minerals interposed between the ends of the rocks that have been ruptured and separated. But in most instances, the opening is filled up by the rise of lava from below. At night, the vents opened on the outside of an active volcano may be traced from afar by the glow of the white-hot lava that rises in them to within a short distance from the surface. When the lava cools and solidifies in these fissures, it forms wall-like masses, known as Dykes (Fig. 43). Inside many volcanic craters, the walls are traversed with dykes which, though on the whole tending to keep a vertical direction, may curve about irregularly according to the form of the vents into which the lava rose. Like the necks above described, dykes form enduring records of volcanic action. The superficial cones and craters may disappear, but the subterranean lava-filled fissures will still remain as records of volcanic action. In some volcanic regions, where enormous floods of lava have been poured forth, no great central cones have existed. Such regions extend as vast black plains of naked rock, mottled with shifting sand-hills, or as undulating tablelands carved by running water into valleys and ravines, between which the successive sheets of lava are exposed in terraced hills. Beyond the limits over which the lava-sheets are spread, dykes of the same kinds of lava rise in abundance to the surface. There can be no doubt that the dykes do not terminate at the edge of the lava-fields, but pass underneath them. Indeed, as they increase in number in that direction, they are probably more abundant underneath the lava than outside of the lava-fields. Sometimes sections are exposed showing how, after rising in a fissure, the lava has spread out on either side as a sheet. In these vast lava-plateaux or deserts, the molten rock, instead of issuing from one main central Etna or Vesuvius, appears to have risen in thousands of fissures opened in the shattered crust, and to have welled forth from numerous vents on these fissures, spreading out sheet after sheet till, like a rising ix LAVA-FLOODS AND DYKES 109 lake, it has not only overflowed the lower grounds, but even buried all the minor hills. Such appears to have been the history of vast tracts in Western North America. The area which has there been flooded with lava has been estimated to be larger than that of France and Great Britain together, and the depth of the total mass of lava erupted reaches in some places as much as 3700 feet. Some rivers have cut gorges in this plain of lava, laying bare its component rocks to a depth of 700 feet or more. Along FIG. 43. Volcanic dykes rising through the bedded tuff of a crater. the walls of these ravines we see that the lava is arranged in parallel beds or sheets often not more than 10 or 20 feet thick, each of which, of course, represents a separate outpouring of molten rock. Except where such deep sections have been cut through them by rivers, recent lava-floods can only be examined along their surface, and we are consequently left chiefly to inference regarding their probable connection with fissures and dykes underneath. But in various parts of the world, lava-plains of much older date have been so deeply eroded as to expose not only the successive sheets of lava but the floor over which they were poured, and the i io VOLCANOES AND EARTHQUAKES CHAP. abundant dykes which no doubt served as the channels wherein the lava rose towards the surface, till it could escape at the lowest levels, or at weaker or wider parts of the fissures. In Western Europe important examples of this structure occur, from the north of Ireland through the Inner Hebrides and the Faroe Islands to Iceland. This volcanic belt presents a succession of lava-fields which even yet, in spite of enormous waste, are in some places more than 3000 feet thick. The sheets of lava are nearly flat, and rise in terraces one over another into green grassy hills, or into the dark fronts of lofty sea-washed precipices. Where this thick cake of lava has been stripped off during the degradation of the land, thousands of dykes are exposed, and many of these traverse at least the lower parts of the sheets of lava. They form, as it were, the subterranean roots of which these sheets were the subaerial branches ; and even where the whole of the material that reached the surface, more than 3000 feet thick, has been worn away, the dykes still remain as evidence of the reality and vigour of the volcanic forces. EARTHQUAKES. The rise of hot springs and the explosions of volcanoes furnish impressive testimony to the internal heat of our planet ; but they are by no means the only proofs that the pent-up energy of the interior of the globe reacts upon the outer surface. By means of delicate instruments, it can be shown that the ground beneath our feet is subject to continual tremors which are too feeble to be perceived by the unaided senses. From these minuter vibrations, movements of increasing intensity can be detected up to the calamitous earthquake, whereby a country is shaken to its founda- tions, and thousands of human lives, together with much valuable property, are destroyed. We do not yet know by what different causes these various disturbances are produced. Some of the fainter tremors may arise from such influences as changes of temperature and atmospheric pressure, and the rise and fall of the tides. But the more violent must be assigned to causes working within the earth itself. The collapse of the roofs of underground caverns, the sudden condensation of steam or explosion of volcanic vapours, the snap of rocks that can no longer resist the strain to which, by the cooling and consequent contraction of the inner hot nucleus, they have been subjected within the earth's crust these . and other influences may at different times come into play to ix TRACES OF EARTHQUAKES in determine convulsive earthquake shocks. Without, however, entering into the difficult question of the causes of the movements, we may inquire into their effects in so far as these register their passing in the annals of geological history. Their awful suddenness and devastation have invested earth- quakes with a high importance in the popular estimate of the forces by which the surface of the globe is modified. Yet if we judge of them by their permanent effects, we must give them a comparatively subordinate place among these forces. After some of the most destructive earthquakes recorded in human history, hardly any trace of the calamity is to be seen, save in shattered and prostrate houses. But when these buildings have been repaired or rebuilt, no one visiting the ground might be able to detect any trace of the earthquake that shattered or overthrew them. Yet severe earthquakes do not pass without their self-written chronicle which, though often evanescent on the face of nature, is at the time conspicuous enough. Landslips are caused, large masses of earth and blocks of rock being shaken down from higher to lower levels ; the ground is rent, and the fissures are sometimes subsequently widened and deepened by rain and runnels into ravines. But more important are the marked changes of level that occasionally accompany earthquake-shocks. In some cases, the ground is raised for several feet, so that along maritime tracts there is a gain of land from the sea ; in others, the ground sinks, and the sea flows in upon the land. Yet it is evident that unless these changes are actually witnessed as the accompaniments of the earthquakes, they may take place without retaining any evidence that they were produced by such a cause. The convulsion of an earthquake, notwithstanding the havoc it may bring to the human population of a country, does not always record itself in distinctive and enduring characters in geological history. Some of its most noticeable effects also are not due directly to its own action, but to the operations of the waters of the land and of the sea which, when disturbed by the shock, not infrequently acquire increased vigour in their own peculiar forms of activity. The great waves set in motion by an earthquake roll over the low lands bordering the sea, and may cause vastly more destruction than is done by the mere shock of the earthquake itself. SLOW UPHEAVAL AND SUBSIDENCE CHAP. UPHEAVAL AND SUBSIDENCE. It is perhaps not so much by earthquakes, as by quiet, hardly perceptible movements, that the relative positions of sea and land are undergoing change at the present time. In some parts of the world the land is gradually rising, in others it is slowly sinking. Proofs of elevation are supplied by lines of barnacles or rock- boring shells, now standing above the reach of the highest tides ; by caves that have obviously been scooped out by the sea, but now stand at a higher level than the waves can reach ; and by deposits of sand, gravel, and shells which were evidently accumu- lated on a beach, but which now rise above the level where similar materials are now being accumulated (Raised Beaches]. Evidences of subsidence are furnished by traces of old land- surfaces trees with roots in situ, and beds of peat, lying below the limits of the tides (Submerged Forests], But it must be more difficult to prove subsidence than elevation, for as the land sinks, its surface is carried below the waves, which soon efface the evidence of terrestrial characters. The time within which man has been observing and recording the changes of the earth's surface forms but an insignificant fraction of the ages through which geological history has been in progress. We cannot suppose that during this brief period he has had experience of every kind of geological process by which the outlines of land and sea are modified. There may be great terrestrial revolutions which happen so rarely that none has occurred since man began to take note of such things. Among these revolutions, of which he has had as yet no experience, the most gigantic is the formation of a mountain-chain. That the various mountain-chains of the globe are of very different ages, and that some of the most gigantic of them are, compared with others, of recent date, are facts in the history of the globe which will be more fully referred to in later pages ; but so far as human history or tradition goes, man has never witnessed the uprise of a range of mountains. The crust of the earth has been folded and crumpled on the most colossal scale, some parts having been pushed for miles away from their original position ; it has been rent by profound fissures, on each side of which the rocks have been displaced for many thousand feet ; and it has been so broken, crushed, and sheared, that its component rocks have in some places assumed a structure entirely different from what they ix SUMMARY 113 originally possessed. But of all these colossal mutations there is no human experience. We are driven to reason regarding them from the record of them preserved among the rocks, and from the analogies that can be suggested by experiments devised to imitate as far as possible the processes of nature. To this subject we shall return in Chapter XIII. Summary. The enduring records left by volcanoes, whence their former existence in almost all regions of the world may be demonstrated, are to be sought partly in the materials which they have brought up to the surface, and partly in the vents and fissures by which they have discharged these materials. Of the former kind of evidence lava furnishes a conspicuous example ; its internal crystalline or glassy structure, its steam-cavities, and the cellular slaggy upper and under parts of the sheets in which it lies, are all proofs of its former molten condition. A succession of lava-beds, piled one above another, marks a series of volcanic eruptions, and the nature of the layers of non-volcanic material intercalated between them may indicate the conditions under which the erup- tions took place, whether on land, in lakes, or in the sea. The fragmentary products consolidated into beds of tuff are likewise characteristic of volcanoes ; they consist mainly of lava-dust with cindery scoriae, slags, and blocks ; they accumulate most deeply and in coarsest material at and immediately around the volcanic vents, but their finer particles may be carried to enormous distances ; they are especially liable to be intercalated with contemporaneous sedimentary deposits in lakes and on the sea-floor. The vents through which lava and ashes are ejected to the surface form the most permanent record of volcanoes, for, being filled up with volcanic rocks to unknown depths, they cannot be destroyed by the mere denudation of the surface, and can only disappear by being buried under later accumulations. Such " necks " consist sometimes of lava, sometimes of consolidated volcanic debris, or of both kinds of material together, and remain as the stumps of volcanoes, where every other trace of volcanic action may have passed away. Not less enduring are the dykes or wall-like masses of lava which have risen and solidified in open fissures. Enormous sheets of lava appear to have flowed out from such fissures in regions where the volcanic energy never produced any great central cone. Earthquakes do not impress their mark upon geological history so indelibly as might be supposed. In spite of the destruction which they cause to human life and property, it is by such direct I ii 4 VOLCANOES AND EARTHQUAKES CHAP, ix changes as landslips, rents of the ground, and the upheaval or depression of land, and by such indirect changes as may be pro- duced by derangements of rivers, lakes, and the sea, that earth- quakes leave their chief record behind them. Some of the most important changes of level now going on are effected quietly and almost imperceptibly, some regions being slowly elevated, and others gradually depressed. But the time within which man has been an observer and recorder of nature is too brief to have supplied him with experience of all the ways in which the internal energy of the globe affects its surface. In particular, he has never witnessed the production of a mountain- chain, nor any of the plications, fractures, and displacements which the crust of the earth has undergone. Regarding these revolutions we can only reason from the records of them in the rocks, and from such laboratory experiments as may seem most closely to imitate the processes of nature that were concerned in their production. PART II ROCKS, AND HOW THEY TELL THE HISTORY OF THE EARTH CHAPTER X THE MORE IMPORTANT ELEMENTS AND MINERALS OF THE EARTH'S CRUST IN the foregoing Part of this volume we have been engaged in considering the working of various processes by which the surface of the earth is modified at the present time, and some of the more striking ways in which the record of these changes is preserved. We have seen that, on the whole, it is by deposits of some kind, laid down in situations where they can escape destruction, that the story of geological revolution is chronicled. In one place it is the stalagmite of a cavern, in another the silt of a lake-bottom, in a third the sand and mud of the sea-floor, in a fourth the lava and ashes of a volcano. In these and countless other examples, materials are removed from one place and set down in another, and in their new position, while acquiring novel characters, they retain more or less distinctly the record of their source and of the conditions under which their transference was effected. In these chapters, reference has intentionally been avoided as far as possible to details that required some knowledge of minerals and rocks, in order that the broad principles of geology, for which such knowledge is not absolutely essential, might be clearly enforced. It is obvious, however, that as minerals and rocks form the records in which the history of the earth has been preserved, this history cannot be followed into detail until some acquaintance with these materials has been made. What now lies before the ii6 ELEMENTS OF EARTH'S CRUST CHAP. reader, therefore, in order that he may be able to apply the know- ledge he has gained of geological processes to the elucidation of former geological periods, is to make himself familiar with at least the more common and important minerals and rocks. This he can only do satisfactorily by handling the objects .themselves, until he acquires such an acquaintance with them as to be able to recognise them where he meets with them in nature. At first the number and variety of these objects may appear to be almost endless, and the learner may be apt to despair of ever mastering more than an insignificant portion of the wide circle of inquiry and observation which they present. But though the detailed study of this subject is more than enough to tax the whole powers of the most indefatigable student, it is not by any means an arduous labour, and assuredly a most interesting one, to acquire so much knowledge of the subject as to be able to follow intelligently the progress of geological investigation, and even to take personal part in it. This accordingly is the task to which he is invited in the present and following chapters. SIMPLE ELEMENTS COMPOSING THE EARTH'S CRUST. Before considering the characters presented by the various rocks that form the visible part of the earth's crust, we may find it of advantage to inquire into the general chemical composition of rocks, for by so doing we learn that though the chemist has detected more than sixty substances which he has been unable to decompose, and which therefore he calls elements^ only a small pro- portion of these enter largely into the composition of the outer part of the globe. In fact, there are only about sixteen elements that play an important part as constituents of rocks ; these together constitute about ninety-nine parts of the terrestrial crust. Half of them are metals ; and the other half are metalloids or non-metals, as in the two subjoined lists, the most abundant being in each case placed first. METALLQIDS OR NON-METALS. METALS. Sy-nbol. AJ Symbol, $g* Oxygen . . O J5-96 Aluminium . Al 27.30 Silicon . . Si 28.00 Calcium . . Ca 39.90 Carbon . . C i:r -97 Magnesium . Mg 23.94 Sulphur . . S 31-98 Potassium . . K 39.04 Hydrogen . . H i.oo Sodium . . Na 22.99 Chlorine . . Cl 35-37 Iron Fe 55.90 Phosphorus . P 30.96 Manganese . Mn 54.80 Fluorine . . F 19.10 Barium . . Ba 136.80 x METALLOIDS OXYGEN, SILICON 117 Some of those elements occur in the free state, that is, not combined with any other element. Carbon, for instance, is found pure in the form of the diamond, and also as graphite. But in the great majority of cases, they assume various combinations. Most abundant are oxides, or compounds of oxygen with another element. Compounds of sulphur and a metal are known as sulphides ; and similar compounds with chlorine are chlorides. Some of the compounds form further combinations with one or more elements. Thus the acid-forming oxides unite with water to form what are called acids, which, combining with metallic oxides or bases, form with them compounds termed salts. Sulphur and oxygen, for example, uniting in certain proportions with water, constitute sulphuric acid (H 2 SO 4 ) which, parting with its displace- able hydrogen and combining with the metal calcium, forms the salt known as calcium-sulphate, or sulphate of lime (CaSO 4 ). METALLOIDS. Of the non-metallic elements, by far the most abundant and important is Oxygen. In its free state, it exists as a gas which has been occasionally detected at active volcanic vents. But with this rare exception, it is always found mixed or combined with one or more elements. Thus, mixed with nitrogen, it con- stitutes the atmosphere, of which it forms not less than 23 per cent by weight. It takes a still larger share in the composition of water, which consists of 88.88 per cent of oxygen and 11.12 of hydrogen. There is a continual removal of oxygen from air and water in the processes of weathering described in Chapter II. Substances which can take more of this element abstract it especi- ally from damp air or from water. A knife or any other piece of iron, for example, will remain unchanged for an indefinite length of time if kept in dry air ; but as soon as it is exposed to moisture, in which there is always some dissolved air, it begins to rust. The familiar brown rust which slowly eats into the very centre of the iron, is due to a chemical union of oxygen with the iron, forming what is called an Oxide of iron with water (see p. 127). Among the rocks of the earth's crust, a large proportion are liable to undergo a similar change, and so enormous has been the extent of this change in the past history of our globe, that some- where about one-half of the outer and accessible part of the crust consists of oxygen, which was probably at first in the atmosphere. Next in importance to oxygen among the metalloids is Silicon, which is never met with in the free state. It has been artificially obtained, however, in the form of a dull brown powder. In nature, it always occurs united with oxygen, forming the familiar substance n8 ELEMENTS OF EARTH'S CRUST CHAP. known as Silica or Silicic Acid (SiO 2 ), which constitutes more than a half of all the known part of the earth's crust. Silica is indeed the fundamental compound of the crust, forming by itself entire masses of rock, and entering as a principal constituent into the majority of rocks. It occurs abundantly as the mineral Quartz, the colourless transparent forms of which are known as rock-crystal (Fig. 44), and also in combination with various metallic bases as the important family of Silicates (p. 1 30). It is present in solution in most natural waters, both those of the land and of the sea, whence it is secreted by plants (diatoms, grasses) and animals FIG. 44. Group of Quartz-crystals (Rock-crystal). (radiolarians, sponges, p. 83). It is thus carried by percolating water into the heart of rocks, and may be deposited in their interstices and cavities. Its hardness and durability eminently fit it for the important part it plays in binding the materials of rocks together, and enabling them better to resist the decompos- ing effects of air and water. Carbon, though found in a nearly pure state in the clear gem called Diamond, and also in the black opaque mineral Graphite, more usually occurs mixed with various impurities, as in the differ- ent kinds of coal. This element has a high importance in nature, because it is the fundamental substance made use of by both plants and animals to build up their structures, and because it serves as a bond of connection between the organic and the x CARBON, SULPHUR 119 inorganic worlds. In union with oxygen, Carbon forms the widely- diffused gaseous compound known as Carbon-dioxide (CO 2 ), which occurs in the proportion of about four parts in every ten thousand parts of ordinary atmospheric air. From the air it is abstracted and decomposed by living plants in presence of sunshine, the oxygen being in great measure sent back into the atmosphere, while the carbon with some oxygen, nitrogen, and hydrogen is built up into the various vegetable cells and tissues. When we look at a verdant landscape or a boundless forest, it is a striking thought that all this vegetation has been chiefly constructed out of the small proportion of invisible carbon-dioxide present in the atmosphere. The vast numbers of beds of coal imbedded in the earth's crust have, in like manner, been derived from the atmo- sphere through the agency of former tribes of plants. Not only in beds of coal, but still more prevalently in masses of limestone, carbon enters into the composition of rocks. Carbon-dioxide, as was pointed out in Chapter II, is abstracted by rain in passing from the clouds to the earth, and is also supplied by decomposing plants and animals in the soil. It is readily dissolved in water, and forms with it carbonic acid, CO(OH) 2 , which has been referred to as so powerful a solvent of the substance of many rocks. This acid unites with a number of alkaline and earthy bases to form the important family of Carbonates. Of these the most abundant is calcium- carbonate, or carbonate of lime (CaCO 3 ), which consists of 44 per cent carbon-dioxide, and 56 per cent lime. This carbonate not only occurs abundantly diffused through many rocks, but in the form of limestone builds up by itself thick mountainous masses of rock many hundreds of square miles in extent. It is abstracted by plants to form calcareous tufa (Chapter V), but far more abun- dantly by animals, especially by the invertebrata, as exemplified by the familiar urchins, corallines, and shells of the sea-shore. The limestones of the earth's crust appear to have been mainly formed of the calcareous remains of animals. Hence we perceive that the two forms in which carbon has been most abundantly stored up in the earth's crust have been principally due to the action of organised life ; coal being chiefly carbon that has been taken out of the atmosphere by plants, and limestone consisting of carbon- dioxide, to the extent of nearly one-half, which has been secreted from water by the agency of animals. Sulphur is found in the free state, more particularly at volcanic vents, in pale yellow crystals or in shapeless masses and grains ; but it chiefly occurs in combination. Some of its compounds are 120 ELEMENTS OF EARTH'S CRUST CHAP. widely diffused among plants and animals. The blackening of a silver spoon by a boiled egg is an illustration of this diffusion, for it arises from the union of the sulphur in the egg with the metal. Combinations with a metal (Sulphides, p. 137) and combinations with a metal and oxygen (Sulphates, p. 136) are the conditions in which sulphur chiefly exists. Hydrogen is a gas which has been detected in the free state at active volcanic vents ; but otherwise it occurs chiefly in combina- tion with oxygen as the oxide water (H 2 O), of which it constitutes about one-ninth, or 11.12 per cent by weight. It also enters into the composition of plant and animal substances, and forms with carbon the important group of bodies known as Hydrocarbons, of which mineral oil and coal-gas are examples. In smaller quantity, it is found united with sulphur (sulphuretted hydrogen, H 2 S), with chlorine (hydrochloric acid, HC1), and a few other elements. Chlorine is a transparent gas of a greenish-yellow colour, but except possibly at active volcanic vents it does not occur in the free state. United with the alkali metals (potassium, sodium, and magnesium), it forms the chief salts of sea-water. The most important of these salts, Sodium-chloride, or common salt (NaCl) contains 60.64 per cent of chlorine, and forms 2.64 per cent by weight of sea-water. This salt is found diffused in microscopic particles in the air, especially near the sea, and beds of it hundreds of feet thick occur in many parts of the world among the sedi- mentary rocks that constitute most of the dry land (pp. 137, 157). Phosphorus does not occur free ; it has so strong an affinity for oxygen that it rapidly oxidises on exposure to the air, and even melts and takes fire. Its most frequent combination is with oxygen and calcium, as Calcium-phosphate or phosphate of lime (Ca 3 (PO 4 ) 2 , p. i 57). Though for the most part present in minute proportions, it is widely diffused in nature. It occurs in fresh and sea water, in soil and in plants, especially in their fruits and seeds ; it is supplied by plants to animals for the formation of bones, which when burnt consist almost entirely of phosphate of lime. Fluorine also is never met with uncombined ; it never unites with oxygen, forming in this respect the sole exception among the elements. Its most frequent combination as a rock constituent is with calcium, when it forms the mineral Fluor-spar (CaF 2 , p. 137). Like phosphorus, it is widely diffused in minute proportions in the waters of some springs, rivers, and the sea, and in the bones of animals. To these metalloids we may add the colourless, tasteless gas x METALS 121 Nitrogen, which, though not largely present in the earth's crust, constitutes four-fifths by volume or 77 per cent by weight of the atmosphere. It does not enter into combination so readily as the other elements above enumerated, but it is always found in the composition of plants and is a constituent of many animal tissues. It is the principal ingredient of the substance called Ammonia, which is produced when moist organic matter is decomposed in the air. In many rocks composed wholly or in great part of organic remains, such, for instance, as peat and coal, nitrogen is a constant constituent. METALS. Though so large a proportion of the known terres- trial elements are metals, these are much less abundant in the earth's crust than the metalloids. The most frequent are Alumi- nium, Calcium, and Magnesium. The substances most familiar to us as metals occupy an altogether subordinate part among rocks, the most abundant of them being Iron. Aluminium never occurs in the free state, but can be artificially separated from its compounds, when it is seen to be a white, light, malleable metal. It is almost always united with oxygen as the oxide of Alumina (A1 2 O 3 ), which occurs crystallised as the ruby and sapphire, but is for the most part united with silica, and in this form constitutes the basis of the great family of minerals known as the Silicates of Alumina, or Aluminous Silicates. These silicates generally contain some other ingredient which is more liable to decomposition, and when they decay and their more soluble parts are removed, they pass into clay, which consists chiefly of hydrated silicate of alumina. Calcium is not met with uncombined, but has been artificially isolated and found to be a light, yellowish metal, between gold and lead in hardness. It occurs in nature chiefly combined with carbonic acid as a carbonate (p. 134), and with sulphuric acid as a sulphate (p. 136), to both of which substances reference has already been made ; it is also present in many silicates. So abundant is Calcium-carbonate or carbonate of lime in nature that it may be found in most natural waters, which dissolve it and carry it in solution into the sea. Its presence in rocks may be detected by a drop of any mineral acid, when the liberated carbon- dioxide escapes as a gas with brisk effervescence. Calcium- sulphate is likewise a common constituent of terrestrial waters, especially of those which in household management are called hard-, it constitutes not less than 3.6 per cent of the salts in ordinary sea-water, and when sea-water is evaporated this sulphate 122 ELEMENTS OF EARTH'S CRUST CHAP. (gypsum, p. 136), being least soluble, is the first to be precipitated in minute crystals resembling in shape those shown in Fig. 62. Magnesium is likewise only isolated artificially, when it appears as a soft, silver-white, malleable and ductile metal. It occurs in sea- water combined with chlorine as Magnesium -chloride, which con- stitutes 10.8 per cent of the total proportion of salts. It unites with carbonic acid as a carbonate, which with carbonate of lime forms the widely diffused rock called magnesian limestone or Dolomite (pp. 135, 155); it also enters into the composition of the Magnesian Silicates which are only second in importance to those of alumina. Potassium and Sodium (alkali metals) are only obtainable in the free state by chemical processes, when they are found to be white brittle metals that float on water, and rapidly oxidise if exposed to the air. Combined with chlorine, Sodium forms the familiar chloride known as common salt, which constitutes 77.7 per cent of the salts of sea-water, is abundantly present in salt lakes, and occurs in extensive beds among the rocks of the dry land (p. 157). Potassium-chloride likewise occurs in the sea and may be obtained from the ashes of burnt sea-weed. Enormous deposits of it, combined with chlorides of sodium and magnesium, have been met with in Germany (Stassfurt). Potassium also exists in the sea in combination with sulphuric acid as potassium-sulphate or sulphate of potash, which amounts to about 2.4 of the total salts of sea-water. Sulphates of potassium, sodium, magnesium, and calcium form thick masses of rock in the Stassfurt deposits. Potassium and sodium in combination with silica form silicates which enter largely into the composition of many rocks. They are readily attacked by water containing carbonic acid, giving rise to what are called carbonates of the alkalies, or alkaline carbonates, which are removed in solution. By this means, carbonate of potash is introduced into soil, where it is taken up by plants into their leaves and succulent parts. When wood is burnt, this carbonate in considerable quantity may be dissolved with water out of the ash. Iron is found in the free or native state in minute grains (rarely in large blocks) in some volcanic rocks, also in granules of " cosmic dust," probably of meteoric origin, and in fragments of various size which have undoubtedly fallen upon the earth's surface from the regions of space. There is reason to believe that much of the solid interior of the globe may consist of native iron and other metals. But it is in combination that iron is chiefly of importance in the earth's crust. It has united with oxygen to form several IMPORTANT MINERALS 123 abundant oxides (p. 128). The protoxide or ferrous oxide (FeO) contains the lowest proportion of oxygen, and being, therefore, prone to take up more, gives rise to many of the processes of decay included under the general name of Weathering (p. 14). It is readily dissolved by organic and other acids, and is then removed in solution, but on exposure rapidly oxidises and passes into the highest oxide, known as the peroxide or sesquioxide of iron or ferric oxide (Fe 2 O 3 ), which, being the permanent insoluble form, is found abundantly among the rocks of the earth's crust. Iron is the great colouring matter of nature ; its protoxide compounds give greenish hues to many rocks, while its peroxide colours them various shades of red which, when the peroxide is combined with water, pass into many tints of brown, orange, and yellow. Manganese is commonly associated with iron in minute pro- portion in many lavas and other crystalline rocks ; its oxides resemble those of iron in their modes of occurrence (p. 1 30). Barium and Calcium are called metals of the alkaline earths. The former can only be obtained in a free state by artificial means, when it appears as a pale yellow very heavy metal which rapidly tarnishes. In nature it chiefly occurs as the sulphate, Barytes, or heavy spar (BaSO 4 ), a mineral of frequent occurrence in veins associated with metallic ores (p. 136). MINERALS OF CHIEF IMPORTANCE IN THE EARTH'S CRUST Passing now from the simple elements, we have next 'to note the mineral forms in which they 'appear as constituents of the earth's crust. A mineral may be defined as an inorganic sub- stance, having theoretically a definite chemical composition, and in most cases also a certain geometrical form. It may consist of only one element, for example, the diamond, sulphur, and the native metals, gold, silver, copper, etc. But in the vast majority of cases, minerals consist of at least two, usually more, elements in definite chemical proportions. In the following short list of the more important minerals of the earth's crust they are arranged chemically, according to the predominant element in them, or the manner in which the combinations of the elements have taken place, so that their leading features of composition may be at once perceived. The two elements, Carbon and Sulphur, in their native or uncombined state, sometimes form considerable masses of rock. Some of the native metals also may be enumerated as rock- constituents when they occur in sufficient quantities to be commer- I2 4 IMPORTANT MINERALS CHAP. cially important. Gold, for example, is found in grains and strings, in veins of quartz, and in irregular pieces or nuggets dispersed through the gravel deposits of regions where gold-bearing quartz- veins traverse the solid rocks. Omitting, however, the minerals formed of a single element, we may pass on to combinations of two or more elements, and consider first those in which oxygen is combined with some other element, forming what are commonly grouped together as Oxides. Then will come the Silicates, or combinations of silica with one or more bases, followed by the Carbonates, or combinations of carbon-dioxide with some base ; the Sulphates, or compounds of sulphuric acid and a base ; the Fluorides, or compounds of fluorine and a metal ; the Chlorides, or compounds of chlorine and a metal ; and the Sulphides, or compounds of sulphur and a metal. It cannot be too strongly impressed upon the mind of the learner that no mere description in books will suffice to make him familiar with minerals and rocks. He ought to handle actual specimens of these objects and identify for himself the several characters which he finds assigned to them in books. One of the most obvious features in a crystal of any mineral is the regular and sharply-defined edges and corners which it presents. Take a piece of rock-crystal or quartz, for example (Fig. 44), and you will find it to consist of six sides or faces, forming what is called a prism, and bevelled off at the end into a six-sided cone, called a pyramid. If you ex- amine a large collection of similar crystals you may find no two of them exactly alike, yet they agree in presenting a six-sided figure. Again, procure a piece of the common mineral calcite, either a whole crystal (Figs. 59, 60), or a portion of a crystalline mass FIG. 45 .-Ca1cite (Iceland spar), showing ( Fi S 45) 5 br eak it and yOU will its characteristic rhombohedral cleavage, find each fragment to pOSSCSS the same form, that of a rhombo- hedron ; crush one of these fragments and you will observe that each little grain of the powder preserves the same shape. The rhombohedron, therefore, is called the fundamental crystalline form of the mineral. The property so strikingly shown in calcite, CRYSTALLINE FORMS OF MINERALS 125 of breaking along definite crystalline planes, is termed cleavage. So perfect is the cleavage of calcite, that the crystallised mineral can hardly be broken, except along the planes that define the rhombohedron. Many minerals cleave more or less easily in one or more directions, and break irregularly in others. The cleavage affords a guide to the proper crystalline form of a mineral. Though there are many hundreds of varieties of crystalline form, they may all be reduced to six primary types or systems. These are distinguished from each other by the number and posi- tion of their a.res, which are mathematical straight lines, intersecting each other in the interior of a crystal, and connecting the centre of opposite flat faces of the crystal, or opposite angles or corners. The six systems, with their axes, are enumerated in the subjoined list. I. Isometric (monometric, cubical, tesseral, regular). In this system there are three axes which are of the same length, and intersect each other at a right angle. The cube, octahedron, and dodecahedron are examples (Fig. 46). Crystals of this system are distinguished by FIG. 46. Cube (a), octahedron (3), dodecahedron (c). their symmetry, their length, breadth, and thickness being equal. Common salt, fluor-spar (Fig. 64), and magnetite (Fig. 54) are illustrations. Tetragonal (dimetric). The axes are three in number, and intersect each other at a right angle, but one of them, called the vertical axis, FIG. 47. Tetragonal prism () and pyramid (a). FIG. 48. Orthorhombic prism. is longer or shorter than the other two, which are lateral axes. Hence a crystal belonging to this 'system may either be oblong or squat (Fig. 47). 126 IMPORTANT MINERALS CHAP. III. Orthorhombic (trimetric) has the three axes intersecting each other at a right angle, but all of unequal lengths. The rectangular and rhombic prisms and the rhombic octahedron belong to this system (Fig. 48). IV. Hexagonal. This is the only system with four axes (Fig. 49). The lateral axes are all equal, intersect at right angles the vertical axis FIG. 49. Hexagonal prism (a), rhombohedron (I/), and scalenohedron (c). (which is longer or shorter than they are), and form with each other angles of 60. Water, for instance, crystallises in this system, and the six-rayed star of a snow-flake is an illustration of the way in which the lateral axes are placed. Quartz is an example (Fig. 44), also calcite (Figs. 59, 60). V. Monoclinic, with all x the axes of unequal length. One of the lateral axes cuts the vertical axis at a right angle, the other intersects the vertical axis obliquely. Augite (Fig. 50), Hornblende (Fig. 57), and Gypsum (Fig. 62) are examples. VI. Triclinic, the most unsymmetrical of all the systems, all the axes being unequal and placed obliquely to each other (Fig. 51). FlG. 50. Monoclinic prism. Crystal of Augite. FIG. 51. Triclinic prism. Crystal of Albite felspar. Every mineral that takes a crystalline form belongs to one or other of these six systems, and through all its varieties of external form the fundamental relations of the axes remain unchanged. Some minerals have crystallised out of solutions in water. How this may take place can be profitably studied by dissolving salt, sugar, or alum in water, and watching how the crystals of these x OXIDES QUARTZ 127 substances gradually shape themselves out of the concentrated solution, each according to its own crystalline pattern. Other minerals have crystallised from hot vapours (sublimation), as may be observed at the fissures of an active volcano (p. 108). Others have crystallised out of molten solutions, as in the case of lava. Thoroughly fused lava is a glassy or vitreous solution of all the mineral substances that enter into the composition of the rock (p. 97), and when it cools, the various minerals crystallise out of it, those that are least fusible taking form first, the most fusible appearing last ; but a residue of non-crystalline glass sometimes remaining even when the rock has solidified (p. 144). It is evident that minerals can only form perfect crystals where they have room and time to crystallise. But where they are crowded together, and where the solution in which they are dis- solved dries or cools too rapidly, their regular and symmetrical growth is arrested. They then form only imperfect crystals, but their internal structure is crystalline, and if examined carefully will be found to show that in the attempt to form definite crystals each mineral has followed its own crystalline type. These characters are of much importance in the study of rocks, for rocks are only large aggregates of minerals, wherein definite crystals are excep- tional, though the structure of the whole mass may still be quite crystalline. But minerals also occur in various indefinite or non-crystalline shapes. Sometimes they are fibrous or disposed in minute fibre- like threads (Fig. 56) ; or concretionary when they have been aggregated into various irregular concretions of globular, kidney- shaped, grape-like, or other imitative shapes (Figs. 61, 64, 65, 75); or stalactitic (Fig. 20) when they have been deposited in pendent forms like stalactites ; or amorphous when they have no definite shape of any kind, as, for instance, in massive ironstone. OXIDES occur abundantly as minerals. The most important are those of Silicon (Quartz) and Iron (Haematite, Limonite, Magnetite, Titanic Iron). Quartz (Silica, Silicic Acid, SiO 2 ), already alluded to, is the most abundant mineral in the earth's crust. It occurs crystallised, also in various crystalline and non-crystalline varieties. In the crystallised form as common quartz it is, when pure, clear and glassy, but is often coloured yellow, red, green, brown, or. black, from various impurities. It crystallises in the six-sided prisms and pyramids above referred to, the clear colourless varieties being rock-crystal (Fig. 44). When purple it is called amethyst ; 128 IMPORTANT MINERALS CHAP. yellow and smoke-coloured varieties, found among the Grampian Mountains, are popularly known as Cairngorm stones. In many places, silica has been deposited as chalcedony, in translucent masses with a waxy lustre, and pale grey, blue, brown, red, or black colours. Deposits of this kind are not infrequent among the cavities of rocks. The common pebbles and agates with concen- tric bands of different colours are examples of chalcedony, and show how the successive layers have been deposited from the walls of the cavity inwards to the centre which is often filled with FIG. 52. Section of a pebble of chalcedony. The outer banded layers are chalcedony, the interior being nearly filled up with crystalline quartz. crystalline quartz (Fig. 52). The dark opaque varieties are called jasper. Quartz can be usually recognised by its vitreous lustre and hardness ; it cannot be scratched with a knife, but easily scratches glass, and it is not soluble in the ordinary acids. It is an essential constituent of many rocks, such as granite and sandstone. Silica being dissolved by natural waters, especially where organic acids or alkaline carbonates are present, is introduced by permeating water into the heart of even the most solid rocks. Hence it is found abundantly in strings and veins traversing rocks, also in cavities and replacing the forms of plants and animals imbedded in sedimentary deposits. Soluble silica is abstracted by some plants and animals and built up into their organic structures (diatoms, radiolarians, sponges). Four minerals composed of Oxides of Iron occur abundantly among rocks. The peroxide is found in two frequent forms, one OXIDES 129 without water (Haematite), the other with water (Limonite). The peroxide and protoxide combine to form Magnetite, and a mixture of the peroxide with the peroxide of the metal titanium gives Titanic Iron. Haematite or Specular Iron (Fe 2 O 3 = Fe7oO3o) occurs in rhombohedral crystals that can with difficulty be scratched with a knife ; but is more usually found in a massive condition with a compact, fibrous, or granular texture, and dark steel-grey or iron-black colour, which becomes bright red when the mineral is scratched or powdered. The earthy kinds are red in colour, and it is in this earthy form that haematite plays so import- ant a part as a colouring material in nature. Red Sandstone, for example, FlG ' 53.-Piece of hematite, showing the , , nodular external form and the internal Owes Its red Colour tO a crystalline structure. deposit of earthy peroxide of iron round the grains of sand. Haematite occurs crystallised in fissures of lavas as a product of the hot vapours that escape at these places ; but is more abundant in beds and concretionary masses (Fig. 53) among various rocks. Limonite or Brown Iron-ore differs from Haematite in being rather softer, in containing more than 14 per cent of water which is combined with the iron to form the hydrated peroxide, in being usually massive or earthy, in presenting a dark brown to yellow colour (ochre), and in giving a yellowish-brown to dull yellow powder when scratched or bruised. It may be seen in the course of being deposited at the present time through the action of vegetation in bogs and lakes (p. 47), hence its name of Bog-iron- ore, likewise in springs and streams where the water carries much sulphate of iron. The common yellow and brown colours of sandstones and many other rocks are generally due to the presence of this mineral. Magnetite (Fe 3 O 4 ) occurs crystallised in octahedrons and dodecahedrons of an iron -black colour, giving a black powder when scratched. It is found abundantly in many rocks (schists, lavas, etc.), sometimes in large crystals (Fig. 54), sometimes in such minute form as can only be detected with the microscope. K 130 IMPORTANT MINERALS CHAP. It also forms extensive beds of a massive structure. Its presence in rocks may be detected by its influence on a magnetised needle. By pounding basalt and some other rocks down to powder, minute crystals and grains of magnetite may be extracted with a magnet. FIG. 54. Octahedral crystals of magnetite in chlorite schist. Titanic Iron (FeTi) 2 O 3 ) occurs in iron -black crystals like those of haematite, from which they may be distinguished by the dark colour and metallic lustre of its surface when scratched. Though it occurs in beds and veins in certain kinds of rock (schists, serpentine, syenite), its most generally diffused condition is in minute crystals and grains scattered through many crystalline rocks (basalt, diabase, etc.) Manganese Oxides are commonly associated with those of iron in rocks. They are liable to be deposited in the form of bog-manganese, under conditions similar to those in which bog- iron" is thrown down. Earthy manganese oxide (wad) not infrequently appears between the joints of fine-grained rocks in arborescent forms that look so like plants as to have been often mistaken for vegetable remains. These plant -like deposits are called Dendrites or dendritic markings (Fig. 55). SILICATES. Compounds of Silica with various bases form by far the most numerous and abundant series of minerals in the earth's crust. They may be grouped according to the chief metallic base in their composition, the most important are the Silicates of Alumina, and the Silicates of Magnesia. Of the aluminous silicates we need consider here only the Felspars, Zeolites, and Mica. Among the magnesian silicates it will be enough to note the leading characters of Hornblende, Augite, Olivine, Talc, Chlorite, and Serpentine. When the learner has made himself so familiar with these as to be able readily to SILICATES recognise them, he may proceed to the examination of others, of which he will find descriptions in treatises on Mineralogy and in more advanced text-books of Geology. Felspars. This family of minerals plays an important part in the construction of the earth's crust, for it constitutes the largest part of the crystalline rocks which, like lava, have been erupted from below; is found abundantly in the great series of schists ; and by decomposition has given rise to the clays, out of which so many sedimentary rocks have been formed. The felspars are divided into two series, according to crystalline form. Orthodase or potash- felspar contains about 16.89 per cent of potash, crystal- lises in monoclinic or oblique rhombic prisms, but also occurs massive ; is white, grey, or pink in colour ; has a glassy lustre ; can with difficulty be scratched with a knife, but easily with quartz. Associated with quartz, it is an abundant ingredient of many ancient crystalline rocks (granite, felsite, gneiss, etc.) In the form of sanidine it is an essential constituent of many modern volcanic rocks. Plagiodase. Under this name are grouped several species of felspar which, differing much from each other in chemical composition, agree in crystallising in the same type or system, which is that of a triclinic or oblique rhomboidal prism. As abundant ingredients of rocks they commonly appear as clear, colourless, or white glassy strips, on the flat faces of which a fine minute parallel ruling may be detected with the naked eye, or with a lens. This striation or lamellation is a distinctive character, which proves the crystals in which it occurs not to be FIG. 55. Dendritic markings due to arboresceot deposit of earthy manganese oxide. *? 132 IMPORTANT MINERALS CHAP. orthoclase. The plagioclase felspars occur as essential constituents of many volcanic rocks, and also among ancient eruptive masses and schists. Among them are Microcline (a potash-felspar), with 1 5 per cent of potash ; Albite or Soda-felspar, containing nearly 1 2 per cent of soda (Fig. 51) ; Anorthite or Lime-felspar, with 20.10 per cent of lime ; Soda-lime felspar, Lime-soda felspar a group of felspars containing variable proportions of soda, lime, and sometimes potash ; the chief varieties are Oligodase (Silica, 62- 65 per cent), Andesine (Silica, 58-61 per cent), Labradorite (Silica, 50-56 per cent). Zeolites, a characteristic family of minerals, composed essen- tially of silicate of alumina and some alkali with water ; often FIG. 56. Cavity in a lava, filled with zeolite which has crystallised in long slender needles. marked by a peculiar pearly lustre, especially on certain planes of cleavage ; usually found filling up cavities in rocks where they have been deposited from solution in water. Some of the species commonly crystallise in fine needles or silky tufts. The zeolites have obviously been formed from the decomposition of other minerals, particularly felspars. They are especially abundant in the steam-cells of old lavas in which plagioclase felspars prevail, either lining the walls of the cavities, and shooting out in crystals or fibres towards the centre (Fig. 56), or filling the cavities up entirely. Mica, a group of minerals (monoclinic) specially distinguished by their ready cleavage into thin, parallel, usually elastic silvery laminae. They are aluminous silicates with potash (soda), or with SILICATES 133 FIG. 57. Horn- blende crystal. magnesia and ferrous oxide, and always with water. They occur as essential constituents of granite, gneiss, and many other eruptive and schistose rocks, also in worn spangles in many sedimentary strata (micaceous sandstone). Among their varieties the two most important are Muscovite (white mica, potash -mica), and Black mica (magnesia-mica, Biotite). Hornblende or Amphibole, a silicate of magnesia, with lime, iron-oxides, and sometimes alumina, occurs in monoclinic (oblique rhombic) prisms, also columnar, fibrous, and massive. It is divisible into (i) a group of pale -coloured varieties, containing little or no alumina, white or pale green in colour, often fibrous (Tremolite, Actinolite, Asbestus), found more particularly among gneisses, marbles, and associated rocks, and (2) a dark group contain- ing 5 to 1 8 per cent of alumina, which replaces the other bases ; dark green to black in colour, in stout, dumpy prisms (Fig. 57), and in columnar or bladed aggregates (Common hornblende). Abundant in many eruptive rocks, and also forming almost entire beds of rock among the crystalline schists. Augite (Pyroxene), in composition resembles hornblende ; indeed, they are only different forms of the same substance, differ- ing slightly in crystalline form, hornblende being the result of slow and augite of rapid crystallisation. Many rocks in which the dark silicate was originally augite have that mineral now replaced by hornblende, as the result of a gradual internal alteration. Like hornblende, augite occurs in two groups : ( I ) pale non-aluminous, found more especially among gneisses, marbles, and as- sociated rocks ; and (2) dark green or black (Fig. 50), occurring abundantly in many eruptive rocks, such as black heavy lavas (basalts, etc.) Olivine (Peridot) (SiO 2 4i.oi, MgO 49.16, FIG 58.-Oli.vme crys- FeO 9.83) occurs in small orthorhombic prisms tal ; the light portions 11 .... 111' / represent the unde- and S^^Y grams in basalts and other lavas ; of composed mineral, the a pale yellowish-green or olive-green colour, shaded parts show the whence its name. These grains can often be conversion of the oli- readily detected on the black ground of the vine into serpentine. ,, -,--,, , i i i > rock, through which they are abundantly dis- persed. Olivine is liable to alteration, and especially to conversion into serpentine by the influence of percolating water (Fig. 58). 134 IMPORTANT MINERALS CHAP. Chlorite (SiO 2 25-28, A1 2 O 3 19-23, FeO 15-29, MgO 13-25, H 2 O 9-12) is a dark olive-green hydrated magnesian silicate. It is so soft as to be easily scratched with the nail, and occurs in small six-sided tables, also in various scaly and tufted aggregations diffused through certain rocks. It appears generally to be the result of the alteration of some previous anhydrous magnesian silicate, such as hornblende. Serpentine (Mg 3 Si 2 O 7 + 2H 2 O) is another hydrated magnesian silicate, containing a little protoxide of iron and alumina, usually massive, dark green but often mottled with red. It occurs in thick beds among schists, is often associated with limestones, and may be looked for in all rocks that contain olivine, of the alteration of which it is often the result. In many serpentines, traces of the original olivine crystals can be detected. CARBONATES. Though these are abundant in nature, only three of them require notice here as important constituents of the earth's crust, those of lime, magnesia and lime, and iron. Calcite (calcium-carbonate, carbonate of lime, CaCO 3 ) crystal- lises in the hexagonal system, and has for its fundamental crystal- FIG. 59. Calcite in the form of "nail-head spar." line form the rhombohedron, as already mentioned (p. 124). When quite pure it is transparent (Iceland spar, Fig. 45), with the lustre of glass ; but more usually is translucent or opaque and white. Its crystals, where the chief axis is shorter than the others, sometimes take the form of flat rhombohedrons (nail-head spar, Fig. 59) ; where, on the other hand, that axis is elongated, they present pointed pyramids (scalenohedrons, dog-tooth spar, Fig. 60). The mineral occurs also in fibrous, granular, and compact forms. The decomposition of silicates containing lime by permeating water gives rise to calcium-carbonate, which is removed in solution. x CARBONATES 135 Being readily soluble in water containing carbonic acid, this carbonate is found in almost all natural waters, by which it is introduced into the cavities of rocks. Some plants and many animals secrete large quantities of carbonate of lime, and their remains are aggregated into beds of limestone, which is a massive and more or less impure form of calcite (pp. 154, 158). Calcite FIG. 60. Calcite in the form of dog-tooth spar. is easily scratched with a knife, and is characterised by its abundant effervescence when acid is dropped upon it. A less frequent and stable form of calcium - carbonate is Aragonite which crystallises in orthorhombic forms, but is more usually found in globular, dendritic, coral- like, or other irregular shapes, and is rather harder and heavier than calcite. Dolomite assumes a rhombohedral crystallisation, and is a compound of 54.4 of magnesium-carbonate, with 45.6 of calcium- carbonate. It is rather harder than calcite, and does not effervesce so freely with acid. It occurs in strings and veins like calcite, but also in massive beds having a prevalent pale yellow or brown colour (owing to hydrated peroxide of iron), a granular and often cavernous texture, and a tendency to crumble down on exposure (p. 155). Siderite (chalybite, spathic iron, ferrous carbonate, FeCO 3 ), another rhombohedral carbonate, contains 62 per cent of ferrous oxide or protoxide of iron. In its crystalline form it is gray or brown, becoming much darker on exposure as the protoxide passes into peroxide. It also occurs mixed with clay in concre- tions and beds, frequently associated with remains of plants and animals (SphfSrofiderite, Clay-ironstone, Figs. 61, 65). 136 IMPORTANT MINERALS CHAP. SULPHATES. Two sulphates deserve notice for their import- ance among rock-masses those of lime and baryta. Gypsum (hydrous calcium-sulphate, CaSO 4 + 2HO 2 ) occurs in monoclinic crystals, commonly with the form of right rhomboidal prisms (Fig. 62, a), which not infrequently appear as macks or twin-crystals (Fig. 62, &). When pure it is clear and colourless, with a peculiar pearly lustre (Selenite) ; it is found fibrous with a silky sheen (Satin-spar}^ also white and granular (Alabaster}. It is so soft as to be easily cut with a knife or even scratched with the finger-nails. It is readily distinguished from calcite by its crystalline form, softness, and non- effervescence with acid. When burnt it becomes an opaque white powder (plaster of Paris). Gyp- sum occurs in beds associated with sheets of rock-salt and dolo- mite (pp. 47, 156); it is soluble in water, and is found in many springs and rivers, as well as in FIG. 6i.-Sph*rosiderite or Clay-ironstone the Sea ' o ne thousand parts of concretion enclosing portion of a fern. water at 32 Fahr. dissolve 2.05 parts of sulphate of lime ; but the solubility of the substance is increased in the presence of common salt, a thousand parts of a saturated solution of common salt taking up as much as 8. 2 parts of the -sulphate. Anhydrous calcium -sulphate or Anhydrite is harder and heavier than gypsum, and is found extensively in beds associated with rock-salt deposits. By absorbing water, it increases in bulk and passes into gypsum. Barytes (Heavy spar, barium -sulphate, BaSO 4 ), the usual form in which the metal barium is distributed over the globe, crystallises in orthorhombic prisms which are generally tabular ; but most frequently it occurs in various massive forms. The purer varieties are transparent or translucent, but in general the mineral is dull yellowish or pinkish white, with a vitreous lustre, and is readily recognisable from other similar substances by its great weight ; it does not effervesce with acids. Barytes is usually met with in veins traversing rocks, especially in association with metallic ores. x PHOSPHATES, FLUORIDES, CHLORIDES, SULPHIDES 137 PHOSPHATES. Only one of these requires to be enumerated in the present list of minerals the phosphate of lime or Apatite. Apatite (tricalcic phosphate, phosphate of lime) crystallises in hexagonal prisms which, as minute colourless needles, are abundant in many crystalline rocks ; it also occurs in large crystals and in amorphous beds associated with gneiss. It is soluble in water containing carbonic acid, ammoniacal salts, common salt, and other salts. Hence its introduction into the soil, and its absorption by plants, as already mentioned (p. 120). FIG. 62. Gypsum crystals. FLUORIDES. The only member of this family occurring con- spicuously in the mineral kingdom is calcium fluoride or Fluor- Spar (Fluorite, CaF 2 ), which, in the form of colourless, but more commonly light green, purple, or yellow cubes, is found in mineral veins not infrequently accompanying lead-ores (Fig. 63). CHLORIDES. Reference has already been made to the only chloride which occurs plentifully as a rock-mass, the chloride of sodium, known as Halite or Rock-salt (NaCl, chlorine 60.64, sodium 39.36). It crystallises in cubical forms, and is also found massive in beds that mark the evaporation of former salt-lakes or inland seas (p. 157). SULPHIDES. Many combinations of sulphur with the metals occur, some of them of great commercial value ; but the only one that need be mentioned here for its wide diffusion as a rock- constituent is the iron-disulphide (FeS 2 ), in which the elements 138 IMPORTANT MINERALS CHAP, x are combined in the proportion of 46.7 iron and 53.3 sulphur. This substance assumes two crystalline forms: (i) Pyrite which occurs in cubes and other forms of the first or monometric system, of a bronze-yellow colour and metallic lustre, so hard as to strike fire with steel, and giving a brownish-black powder when scratched. This mineral is abundantly diffused in minute grains, strings, veins, concretions (Fig. 64, c), and crystals in many different kinds of rocks ; it is usually recognisable by its colour, FIG. 63. Group of fluor-spar crystals. lustre, and hardness j (2) Marcasite (white pyrite) crystallises in the tetragonal system, has a paler colour than ordinary pyrite, and is much more liable to decomposition. This form, rather than pyrite, is usually associated with the remains of plants and animals imbedded among rocks. The sulphide has no doubt often been precipitated round decaying organisms by their effect in reducing sulphate of iron. By its ready decomposition, marcasite gives rise to the production of sulphuric acid and the consequent formation of sulphates. One of the most frequent indications of this decomposition is the rise of chalybeate springs (P- 59). CHAPTER XI THE MORE IMPORTANT ROCKS OF THE EARTH'S CRUST FROM the distribution of the more important elements in the earth's crust and the mineral forms which they assume, we have now to advance a stage farther and inquire how the minerals are combined and distributed so as to build up the crust. As a rule, simple minerals do not occur alone in large masses ; more usually they are combined in various proportions to form what are known as Rocks. A rock may be defined as a mass of inorganic matter, composed of one or more minerals, having for the most part a variable chemical composition, with no necessarily symmetrical external form, and ranging in cohesion from loose or feebly aggregated debris up to the most solid stone. Blown sand, peat, coal, sandstone, limestone, lava, granite, though so unlike each other, are all included under the general name of Rocks. In entering upon the study of rocks, or the division of geology known as Petrography, it is desirable to be provided with such helps as are needed for determining leading external characters ; in particular, a hammer to detach fresh splinters of rock, a pocket- knife for trying the hardness of minerals, a small phial of dilute hydrochloric acid for detecting carbonate of lime, and a pocket lens. The learner, however, must bear in mind that the thorough investigation of rocks is a laborious pursuit, requiring qualifications in chemistry and mineralogy. He must not expect to be able to recognise rocks from description until he has made good progress in the study. As already stated on a previous page, he must examine the objects themselves, and for this purpose he will find much advantage in procuring a set of named specimens, and making himself familiar with such of their characters as he can himself readily observe. Great light has in recent years been thrown upon the structure HO ROCKS OF EARTH'S CRUST CHAP. and history of rocks by examining them with the microscope. For this purpose, a thin chip or slice of the rock to be studied is ground smooth with emery and water, and after being polished with flour-emery upon plate-glass, the polished side is cemented with Canada balsam to a piece of glass, and the other side is then ground down until the specimen is so thin as to be transparent. FIG. 64. Concretions. a, b, " Fairy stones ;" c, Pyrite, showing internal radiated structure. Thin sections of rock thus prepared (which can now be obtained from any good mineral-dealer) reveal under the microscope the minutest kinds of rock-structure. Not only can the component minerals be detected, but it is often possible to tell the order in which they have appeared, and what has been the probable origin and history of the rock. Some illustrations of this method of investigation will be given in a later part of the present chapter. It will be of advantage to begin by taking note of some of the more important characters of rocks, and of the names which geologists apply to them. TERMS APPLIED TO ROCKS 141 Some important Terms applied to Rocks. Sedimentary composed of sediment which may be either a mechanically suspended detritus, such as mud, sand, shells, or gravel ; or a chemical precipitate, as rock-salt and calcareous tufa. The various deposits which are accumulated on the floors of lakes, in river-courses, and on the bed of the sea, are examples of sedi- mentary rocks. Fragment^ Clastic composed of fragments derived from some previous rock. All ordinary detritus is of this nature. Concretionary composed of mineral matter which has been aggregated round some centre so as to form rounded or irregularly- shaped lumps. Some minerals, particularly pyrite (Fig. 64 c), marcasite, siderite, and calcite, are frequently found in concretion- ary forms, especially round some organic relic, such as a shell or plant (Figs. 61, 65). In alluvial clay, calcareous concretions which often take curious imitative shapes, are known as "fairy stones " (Fig. 64, a, b\ see p. 177). When nodules of limestone, ironstone, or cement-stone are marked internally by cracks which radiate towards, but do not FIG. 65. Section of a septarian nodule, with coprolite of a fish as a nucleus. reach, the outside, and are filled up with calcite or other mineral, they are known as Septaria or septarian nodules (Fig. 65, and layer 13 in Fig. 80). Oolitic made up of spherical grains, each of which has been 142 ROCKS OF EARTH'S CRUST CHAP. formed by the deposition of successive coatings of mineral matter round some grain of sand, fragment of shell, or other foreign particle (Fig. 66). A rock with this structure looks like fish-roe, FIG. 66. Piece of oolite. hence the name oolite or roe-stone ; but when the granules are like peas, the rock becomes pisolitic (pea-stone, Fig. 67). This peculiar structure is produced in water (springs, lakes, or enclosed FIG. 67. Piece of pisolite. parts of the sea), wherein dissolved mineral matter (usually carbonate of lime) is so abundant as to be deposited in thin pellicles round the grains of sediment that are kept in motion by the current (p. 88). Stratified, Bedded arranged in layers, strata, or beds lying XI TERMS APPLIED TO ROCKS generally parallel to each other, as in ordinary sedimentary deposits (Fig. 79, p. i? 2 )- Aqueous laid down in water, comprising nearly the whole of the sedimentary and stratified rocks. Unstratified, Massive having no arrangement in definite layers or strata. Lavas and the other eruptive rocks are examples (Chapter XIV). Eruptive, Igneous forced upwards in a molten or plastic condition into or through the earth's crust. All lavas are Eruptive or Igneous rocks, also called Volcanic because erupted to the surface by volcanoes. In the same division must be classed granite and allied masses, which have been thrust through rocks at some depth within the earth's crust and may not have been directly connected with any volcanic eruption ; such rocks are sometimes called Plutonic or Hypogene. Crystalline consisting wholly or chiefly of crystals or crystal- line grains. Rocks of this nature may have arisen from (a) igneous fusion, as in the case of lavas, where the minerals have separated out of a molten glass, or what is called a Magma ; (&) aqueous solution, as where crystalline calcite forms stalactite and stalagmite in a cavern ; (c) sublimation, where the materials have crystallised out of hot vapours, as in the vents and clefts of volcanoes. By the aid of the microscope many rocks which to the naked eye show no definite structure can be shown to be wholly or partially crystalline. Moreover, it can often be ascertained that the crystals or crystalline grains in a rock, as they were crystal- lising out of their solution, have en- closed various foreign bodies. Among the objects thus taken up are minute globules of gas, which are prodigi- ously abundant in certain minerals in some lavas ; liquids, usually water, enclosed in cavities of the crystals, but not quite filling them, and leaving a minute freely-moving bubble (Fig. 68) ; glass, filling globular spaces, probably part of the original glassy magma of the rock ; crystals and crystallites (rudimentary crystalline forms, Fig. 69) of other minerals. Thus a crystal, which, to the eye may appear quite free from impurities, may be found to be full of various kinds of enclosures. Obviously the study of these en- FIG. 68. Cavities in quartz con- taining liquids (magnified). 144 ROCKS OF EARTH'S CRUST CHAP. closures cannot but throw light on the conditions under which the rocks enclosing them were produced. There are various types of crystalline structure which can best be examined under the microscope, as Holocrystalline, com- posed entirely of crystalline elements without any interstitial glass one of the most characteristic types of this structure is found in V >-**- /ix FIG. 69. Various forms of crystallites (highly magnified). granite, hence it is sometimes termed the granitic or granitoid structure ; Semi-crystalline, consisting partly of crystals, but with a ground mass or base which may be partly glassy or variously devitrified ; Felsitic or microfelsitic composed of indefinite half- effaced granules and filaments (p. 163). Glassy^ Vitreotis having a structure and aspect like that of artificial glass. Some lavas, obsidian for example, have solidified as natural glasses, and look not unlike masses of dark bottle-glass. In almost all cases, however, they contain dispersed crystals, crystallites, or other enclosures. These substances have generally multiplied to such an extent in most lavas as to leave only small interstitial portions of the original glass, while in many cases the glass has entirely disappeared. When a glass has thus been con- verted into a dull, opaque, stony, or lithoid mass, or into a com- pletely crystalline substance, it is said to be devitrified. The microscope enables us to prove many crystalline eruptive rocks to have been once molten glass which by a process of devitrifica- tion have been brought into their present more or less crystalline condition (p. 97). Porphyritic composed of a compact or crystalline base or matrix, through which are scattered conspicuous crystals much larger than those of the base, and generally of some felspar. Many eruptive rocks have this structure and are sometimes spoken of as " porphyries." The large crystals existed in the rock while still in a mobile state within the earth's crust, while the minuter crystals of the base were developed by a later process of crystallisa- XI TERMS APPLIED TO ROCKS tion during the consolidation of the rock. In the successive zones of growth which porphyritic crystals often present, we may note by the enclosed minerals some of the successive stages of consolidation. FIG. 70. Porphyritic structure. Spherulitic composed of or containing small pea-like globular bodies (Spherulites) which show a minutely fibrous internal struc- ture radiating from the centre (Fig. 71, A). This structure is A B FIG. 71. Spherulites and fluxion-structure. A, Spherulites, as seen under the microscope (with polarised light). B, Fluxion-structure of Obsidian, as seen under the microscope. particularly observable in vitreous rocks, where it appears to be one of the stages of devitrification (p. 144). Perlitic. Many vitreous rocks show a minute fissured structure as one of the accompaniments of devitrification. In the structure termed perlitic the original glass has had a series of reticulated and globular or spiral cracks developed in it, sometimes giving rise to globules composed of successive thin shells. 146 ROCKS OF EARTH'S CRUST CHAP. Vesicular, Cellular containing spheroidal or irregularly shaped cavities. In many eruptive rocks (as in modern lavas) the ex- pansion of interstitial steam, while the mass was still in a molten condition, has produced this cellular structure (Fig. 35), the vesicles have usually remarkably smooth walls ; they may form a comparatively small part of the whole mass, or they may so increase as to make pieces of the rock capable of floating on water. Where the vesicular structure is conjoined with more solid parts, as in the irregular slags of an iron furnace, it may be called slaggy. Where, as in the scoriae of a volcano, the cellular and solid parts are in about equal proportions, and the vesicles vary greatly in numbers and size within short distances, the structure may be termed scoriaceous. The lighter and more froth- like varieties that can float on water are said to be pumiceous, or to have the characters of pumice (p. 162). Exposed to the influence of percolating water, vesicular rocks have had their vesicles filled up by the deposition of various minerals from solution, especially quartz, calcite, and zeolites. These substances first begin to encrust the walls of the cells, and as layer succeeds layer they gradually fill the cells up (Fig. 52) ; as the cells have not infrequently been elongated in one direction by the motion of the rock before consolidation was completed (Fig. 37), the mineral deposits in them, taking their exact moulds, appear as oval or almond-shaped bodies. Hence rocks which have been treated in this way are called Amygdaloids, and the kernels filling up the cells are known as Amygdules (Fig. 35). An amyg- daloidal rock, therefore, was originally a molten lava, rendered cellular by the expansion of its absorbed steam and gases, its vesicles having been subsequently filled up by the deposit in them of mineral matter, often derived ou of the surrounding rock by the decomposing and rearranging action of percolating water. Flow-structure, Fluxion-structtire an arrangement of the crystallites, crystals, or particles of a rock in streaky lines, the minuter forms being grouped round the larger, indicative of the internal movement of the mass previous to its consolidation. The lines are those in which the particles flowed past each other, the larger crystals giving rise to obstructions and eddies in the movement of the smaller objects past them. This structure is characteristic of many once molten rocks ; it is well seen in obsidian (Fig. 71, B). But it is also found in rocks which, by enormous stresses within the earth's crust, have been crushed and made to undergo an interstitial movement like that of the flow of xi DESCRIPTIVE TERMS CLASSIFICATION 147 liquids. The most solid gneisses and granites have in this way been so sheared and squeezed that their component minerals have been crushed into a fine compact mass, through which the streaking lines of flow are sometimes displayed with singular clearness. Mylonitic a name sometimes applied to rocks which by terrestrial movement have had their original structure entirely obliterated, and which now present only a dull, crushed felsitic mass, sometimes partially or completely recrystallised. Schistose, Foliated consisting of minerals that have crystal- lised in approximately parallel, wavy, and irregular laminae, layers, FIG. 72. Schistose structure. or folia (Fig. 72). Such rocks are called generally schists. They have, in large measure, been formed by the alteration or metamorphism of other rocks of various kinds by the vast terres- trial movements referred to in the foregoing paragraphs (see Chapter XIII). CLASSIFICATION OF ROCKS Various schemes of classification of rocks are in use among geologists, some based on mode of origin, others on mineral composition or structure. For the purpose of the learner, perhaps the most instructive and useful arrangement is one which as far as possible combines the advantages of both these systems. Accordingly, in the following account of the more important rocks which enter into the structure of the earth's crust, a threefold i 4 8 ROCKS OF EARTH'S CRUST CHAP. subdivision will be adopted into : (i) sedimentary rocks ; (ii) eruptive rocks ; (iii) schistose rocks. I. SEDIMENTARY ROCKS. This division includes the largest number, and to the geologist the most important of the rocks accessible to our notice. It comprises the various deposits that arise from the decay of the surface of the land and are laid down on the ' id or over the bed of the sea, together with all those directly or indirectly due to the growth of plants and animals. It thus embraces those which constitute the main mass of the earth's crust so far as known to us, and which contain the evidence whence the geological history of the earth is chiefly worked out. It is, therefore, worthy of the earliest and closest attention of the student. Sedimentary rocks, being due to the deposition of some kind of sediment or detritus, are obviously not original or primitive rocks. They have all been derived from some source, the nature of which, if not its actual site, can usually be determined. In no case, therefore, can sedimentary rocks carry us back to the beginning of things ; they are themselves derivative and pre- suppose the existence of some older rock or material from which they could be derived. One of their most obvious characters is that, as a rule, they are stratified. They have been deposited, usually in water, some- times in air, layer above layer, and bed above bed, each of these strata marking a particular interval in the progress of deposition (Chapter XII). As regards their mode of origin, they may be subdivided into three great sections: (i) fragmental or clastic, composed of fragments of pre-existing rocks ; (2) chemically pre- cipitated, as in the deposits from mineral springs ; and (3) formed of the remains of organisms, as in peat and coral-rock. ( i ) Fragmental or Clastic Rocks. These are masses of mechanically-formed sediment, derived from the destruction of older rocks ; they vary in coherence from loose sand or mud up to the most compact sandstone or con- glomerate ; they are accumulating abundantly at the present time in the beds of rivers and lakes, and on the floor of the sea, and they have been formed in a similar way all over the globe from the earliest periods of known geological history. Some of the more frequent kinds are the following : xi FRAGMENTAL SEDIMENTARY ROCKS 149 Cliff-Debris coarse angular rubbish, including large blocks of stone, disengaged by the weather from cliffs and other bare faces of rock. This kind of detritus is formed abundantly in rugged and mountainous regions, especially where the action of frost is severe ; it slides down the slopes and accumulates at their foot, unless washed away by torrents. In glacier-valleys it descends to the ice, where, gathering into moraines (Chapter VI), it is transport ~A , to lower levels. The perched blocks of such valleys are some of the larger fragments of this cliff-debris left stranded by the ice, and from around which the smaller detritus has been washed away (Fig. 23). Soil, Subsoil, described in Chapter II, represent the result of the subaerial decomposition of the surface of the land. Breccia a rock composed of angular fragments. Such a rock shows that its materials have not travelled far ; otherwise, they FIG. 73. Brecciated structure volcanic breccia, a rock composed of angular fragments of lava, in a paste of finer volcanic debris. would have lost their edges, and would have been more or less rounded. Ordinary cliff-debris may consolidate into a breccia, more especially where it falls into water and is allowed to gather on the bottom. The angular fragments shot out of a volcano often accumulate into volcanic breccia (Fig. 73). A rock with abundant angular fragments is said to be brecciated. Gravel loose rounded water -worn detritus, in which the pebbles range in average size between that of a small pea and that of a walnut ; where they are' larger they form Shingle. They may ISO ROCKS OF EARTH'S CRUST CHAP. consist of fragments of any kind of rock, though having resulted from more or less violent water-action, as a rule, pieces of only the more durable stones are found in them. Quartz and other siliceous materials, from their great hardness, are better able to withstand the grinding to which the detritus on an exposed sea- shore, or in the bed of a rapid stream, is subjected. Hence quartzose and siliceous pebbles are the most frequent constituents of gravel and shingle. Conglomerate a name given to gravel and shingle when they have been consolidated into stone, the pebbles being bound to- gether by some kind of paste or cementing material, which may FIG. 74. Conglomerate. be fine hardened sand, clay, or some calcareous, siliceous, or ferruginous cement (Fig. 74). As above remarked with regard to gravel, the component materials of conglomerate may have been derived from any kind of rock, but siliceous pebbles are of most common occurrence. Different names are given to conglomerates, according to the nature of the pebbles, as quartz-conglomerate, flint-conglomerate, limestone-conglomerate. Sand a name given to fine kinds of detritus, the grains of which may vary from the size of a small pea down to- minute particles that can only be detected with a lens. In general, for the reason already assigned in the case of gravel, the component grains of sand are of quartz or of some other durable material. Examined with a good magnifying glass, they are seen to be usually rounded, water-worn, but sometimes angular, unworn particles of indefinite shapes which, except in their smaller size, xi FRAGMENTAL SEDIMENTARY ROCKS 151 resemble those of gravel-stones. Sand may be formed by the disintegration of the surface of rocks exposed to the weather, more especially in dry climates, where there is a great difference between the temperature of day and night (p. 1 3). The loosened particles are blown away by the wind, and may be heaped up into great sand-wastes, as in the tracts known as Deserts. On a sea-coast, where a sandy beach is liable to be laid bare and exposed to be dried between tides by breezes blowing from the sea, the upper particles of sand are lifted up by the wind and borne away land- ward, to be piled up into dunes (p. 20). In some places the materials are derived mainly from the remains of calcareous sea-weeds, shells, corallines, and other calcareous organisms exposed to the pounding action of the surf. A sand composed of such materials speedily hardens into a more or less coherent and even compact limestone, for rain falling on it dissolves some carbonate of lime which, being immediately deposited again, as the moisture evaporates, coats the grains of sand and cements them together. At Bermuda, as already stated, all the rock above sea-level has been formed in this way, and some of it is hard enough to make a good building stone (p. 84). Ordinary siliceous or quartzose sand remains loose, unless its grains are made to cohere by some kind of cement, when it becomes sandstone. Sandstone consolidated sand. The grains are chiefly quartz, but may include particles of any other mineral or rock ; they are bound together by some kind of cement which has either been laid down with them at the time of their deposition, or has subse- quently been introduced by water permeating the sand. The cementing material may be argillaceous that is, some kind of clay; or calcareous, consisting of carbon ale_oflirae ; ^ferruginous^ composed mainly of peroxide of iron ; or siliceous^ where silica has been deposited in the interstices of the mass. The colours of sandstone vary chiefly with the nature of this cementing material. The hydrous peroxide of iron colours them shades of yellow and brown ; the anhydrous peroxide of iron gives them different hues of red ; the mineral glauconite tints them a greenish hue. Some varieties of sandstone are named after a conspicuous component or structure ; thus micaceous sandstone is distinguished by abundant spangles of mica deposited along the bedding planes, whereby the rock can be split up into thin layers ; freestone a thick-bedded sandstone that does not tend to split up in any one direction, and can therefore be cut into blocks of any size and form ; glauconitic sandstone (green sand), containing green grains 152 ROCKS OF EARTH'S CRUST CHAP. and kernels of glauconite ; quartzose sandstone, conspicuously composed of quartz-grains ; grit a sandstone formed of coarse or sharp, somewhat angular grains of quartz. Greywacke a greyish, compact, granular rock, composed of rounded or subangular grains of quartz and other minerals or rocks, cemented together in a compact paste ; it differs from sandstone chiefly in its darker colour, in the proportion of other grains than those of quartz, and in the presence of a tough cement. The rocks above enumerated represent the coarser and more durable kinds of detritus derived from the weathering of the sur- face of the land ; but during the progress of the decomposition from which these materials are derived some of the component ingredients of the rocks decay into clay, or what is called argil- laceous sediment. This more particularly occurs in the case of felspars and other aluminous silicates, the decomposition of which produces minute particles capable of being lifted up and carried a great distance by running water. Hence argillaceous sediment, being commonly finer in grain, travels farther, on the whole, than quartzose sediment ; and beds of clay denote, generally, deeper and stiller water than beds of sand. Clay a fine-grained argillaceous substance, derived from the decay and hydration of aluminous silicates, white when pure, but usually mixed with impurities, which impart to it various shades of grey, green, brown, red, purple, or blue ; it usually contains interstitial water, and when wet can be kneaded between the fingers ; when dry it is soft and friable, and adheres to the tongue. Shaken with water it becomes Mud\ even a small quantity will make a glass of water turbid, so fine are the particles of which it is composed. Kaolin the name given to the white purer forms of clay, resulting from the decomposition of the felspars of granite or similar rocks ; it is sometimes called China-clay, from its use in the manufacture of porcelain. Fire-clay a white, grey, yellow, or black clay, nearly free from alkalies and iron, and capable of standing a great heat without fusing ; it is abundantly found underneath coal-seams, where it represents the ancient soil on which the plants grew that have been converted into coal. Brick-clay- a name commonly applied to any clay, loam, or earth from which bricks can be made. Such deposits are always more or less sandy and impure clays ; in the south of England XI FRAGMENTAL SEDIMENTARY ROCKS 153 they have largely arisen from the prolonged subaerial waste of the Cretaceous and Tertiary formations. Mudstone a compact solidified clay or clay-rock, having little or no tendency to split into thin laminae. Shale clay that has become hard and splits into thin laminae which lie parallel with the planes of deposit (p. 1 72). A thoroughly fissile shale can be subdivided into leaves as thin as fine cardboard. This is the common form which the clays of the older geological formations have assumed. Gradations can be traced from shale into other sedimentary rocks ; thus, by additions of sand into fissile sandstones, of calcareous cement into limestone, of carbon- ate of iron into ironstone, of carbonaceous matter into coal. These passages are interesting as indications of the conditions under which the rocks were formed. Where, for example, shale shades off into coral-limestone, we see that mud gathered over one part of the sea-floor, while not far off, probably in clearer water, corals flourished and built up a limestone out of their remains (see p. 1 79). Loess a pale somewhat calcareous and sandy clay, found in regions where it has probably been accumulated by the drifting action of the wind. It is sufficiently coherent to be capable of excavation into tunnels and passages, and in China is even dug out into houses and subterranean villages. It occupies parts of the valleys of the Rhine, Danube, Mississippi, and other large rivers, but also crosses watersheds (p. 363). Fragmental rocks of volcanic origin may be enumerated here. They consist partly of materials ejected in fragmentary form from volcanic vents, and partly of the detritus derived from the disin- tegration of volcanic rocks already erupted to the surface. They are comprised under the general name of Tuff($. 101). Bombs round elliptical or discoidal pieces of lava which have been ejected in a molten state from an active vent, and have acquired their form from rapid rotation in the air during ascent and descent. They are often very cellular or even quite empty inside. Where the large ejected stones are of irregular forms, and appear to have been thrown out in an already solidified con- dition, as from the consolidated crust of the lava-plug, or from the sides of the funnel or crater, they are called Volcanic Blocks (p. 101). Lapilli ejected pieces of lava, usually vesicular or porous, from the size of a pea to a walnut (Fig. 73). Volcanic Ash the fine dust produced by the explosion of the 154 ROCKS OF EARTH'S CRUST CHAP. superheated steam absorbed in molten lava. Under the micro- scope, it is often found to consist of minute grains of glass, and in such cases, shows that the lava from which it was derived, rose from below in the condition of a liquid glassy magma. In other instances, it is made up of the crystallites and crystals that arose during the devitrification of the glass. It consolidates into a more or less coherent mass, which is known as Tujf, and which may receive some distinctive name according to the nature of the lava that has supplied it, as Basalt-tuff 'and Trachyte-tuff. Most tuffs contain angular and vesicular pieces of lava, and sometimes pass into coarse breccias (Volcanic Breccia). In many cases, they enclose the remains of plants and animals which, if of terrestrial kinds, indicate that the eruptions took place on land ; if of marine species, that the volcanoes were probably submarine (pp. 101-103). Agglomerate a coarse, usually unstratified accumulation of blocks of lava and other rocks, not infrequently filling up the chimney or neck of a volcanic vent. (2) Rocks formed by Chemical Precipitation. In Chapter V it was pointed out that all natural waters contain in solution invisible mineral matter which they have dissolved out of the rocks of the earth's crust, and that the quantity of this material is sometimes so great that it is precipitated into visible form as the water evaporates. The substance most abundantly dissolved and deposited is Carbonate of lime. Others of fre- quent occurrence are Sulphate of lime, Chloride of sodium, Silica, Carbonate of magnesia, and various salts of iron. Among the rocks of the earth's crust, considerable masses of these substances have been piled up by chemical precipitation. Limestone compact or crystalline calcium-carbonate (carbon- ate of lime) which may be nearly pure, or may contain sand, clay, or other impurity, and may consequently pass into sandstone, shale, or other sedimentary rock. Probably the great majority of the limestones in the earth's crust have been formed by the agency of animals, as more particularly referred to at p. 158. We are here concerned only with those which have been deposited from chemical solution. The most familiar example of this kind of limestone is afforded by stalactites and stalagmite, which have already been described (Chapter V and Fig. 20). Large masses of it have been deposited by calcareous springs and streams (p. 57). xi CHEMICALLY PRECIPITATED ROCKS 155 At first, it is a fine white milky precipitate, but gradually crystals of calcite shape themselves and grow out of it, with their vertical axes usually at right angles to the surface of deposit. In a vertical stalactite, consequently, the prisms radiate horizontally from the centre outwards ; on a horizontal surface of stalagmite they diverge perpendicular to the floor. A mass of limestone, not originally crystalline, may thus acquire a thoroughly crystalline internal structure by the action of infiltrating water in dissolving the carbonate of lime and redepositing it in a crystalline condition. Limestones vary greatly in texture and purity. Some are snow-white and distinctly crystalline ; others are grey, blue, yellow, or brown, dull and compact, and full of various impurities. They may usually be detected by the ease with which they can be scratched, and their copious effervescence when a drop of weak acid is put on the scratched surface. Pure limestone dissolves entirely in hydrochloric acid, so that the amount of residue is an indication of the proportion of insoluble impurity. Among the varieties of limestone the following may be named : Oolite, a limestone composed of minute spherical grains like the roe of a fish, each grain being composed of concentrically deposited layers or shells of calcite (Fig. 66) ; Pisolite, a .similar rock, where the grains are as large as peas (Fig. 67) ; Travertine or calcareous tufa, a white porous crumbling rock which, by infiltration of carbonate of lime, may acquire a compact texture, and become suitable for building stone (p. 57); Hydraulic lime- stone, containing 10 to 30 per cent of fine sand or clay, and having the property, after being burnt, of hardening under water into a firm compact mortar. Dolomite, Magnesian Limestone this substance has been already referred to as a mineral (p. 135); but it also occurs in large masses as a white or yellowish crystalline or compact rock. The white varieties look like marble. The yellow and brown kinds contain various impurities, and are coloured by iron-oxide. Dolomite differs from limestone in its greater hardness and feebler solubility in acid, in its frequently cellular or cavernous texture, in its tendency to assume spherical, grape-shaped, or other irregular concretionary forms (Fig. 75), and in its prone- ness to crumble down into loose crystals. It occurs in beds, not uncommonly associated with gypsum and rock-salt, and in such conditions it may have been deposited Tirst as limestone which, by the chemical action of the magnesian salts in the saline water, had its carbonate of lime partially replaced by carbonate of 156 ROCKS OF EARTH'S CRUST CHAP. magnesia. It is also found in irregular bands traversing lime- stone which, probably by the influence of percolating water containing carbonate of magnesia in solution, has been changed into dolomite. Gypsum is not only a mineral (p. 136) but also a rock, white, grey, brown, or reddish in colour, granular to compact, some- FIG. 75. Concretionary forms assumed by Dolomite, Magnesian Limestone, Durham. times fibrous or coarsely crystalline in texture. It consists of sulphate of lime, is easily scratched with the nail, and is not affected by acids, being thus readily distinguishable from lime- stone. It is found in beds or veins, especially associated with layers of red clay and rock-salt, and in these cases has evidently resulted from the evaporation of water containing it in solution, such as that of the sea. The lime-sulphate being less soluble than the other constituents is precipitated first. Hence in a thick series of alternations of beds of gypsum (or anhydrite) and rock-salt, each layer of sulphate of lime indicates a new supply of water into the natural reservoirs where the evaporation took place; The overlying bed of salt, usually much thicker than the gypsum, points to the condensation of the water into a strong brine, from xi CHEMICALLY PRECIPITATED ROCKS 157 which the salt was ultimately precipitated. And the next sheet of sulphate of lime tells how, by the breaking down of the barrier, renewed supplies of salt water were poured into the basin (pp. 47, Rock-salt occurs in beds or layers, from less than an inch to hundreds or even thousands of feet in thickness. One mass of salt in Galicia is more than 4600 feet thick, and a still thicker mass occurs near Berlin. When quite pure, rock-salt is clear and colourless, but it is usually more or less mixed with impurities, particularly with red clay, and in association with beds of gypsum, as above remarked. It has been formed in inland salt lakes or basins by the evaporation and concentration of the saline water. It is being deposited at the present time in the Dead Sea, the Great Salt Lake, and the salt lakes so frequent in the desert regions of continents, where the drainage does not flow outwards to the sea (p. 47). Ironstone. Various minerals are included under this name as large rock-masses. One of the most important of them is Hcematite (p. 129), which occurs in large beds and veins, as well as rilling up caverns in limestone. Limonite or bog-iron ore is formed in lakes and marshy places (p. 47), and occurs in beds among other sedimentary accumulations. Magnetite (p. 129) is found in beds and huge wedge-shaped masses among various crystalline rocks, as in Scandinavia, where it sometimes forms an entire mountain. Carbonate of iron (Siderite, Sphaerosiderite, Clay-ironstone) occurs in concretions and beds among argillace- ous deposits (Figs. 61, 65, and p. 135). In the Coal-measures, for example, it is largely developed, much of the iron of Britain being obtained from this source. As many ironstones are largely due to the influence of plants and animals, the rock -is alluded to again on p. 160. Siliceous Sinter a white powdery to compact and flinty deposit from the hot water of springs in volcanic districts, con- sisting of 84 to 91 per cent of silica, with small proportions of alumina, peroxide of iron, lime, magnesia, and alkali, and from 5 to 8 per cent of water. It accumulates in basin-shaped cavities round the mouths of hot springs and geysers, and sometimes forms extensive terraces and mounds, as at the geyser regions of Iceland, Wyoming, and New Zealand. Vein-quartz a massive form of quartz, which occurs in thin veins and in broad dyke-like reefs, traversing especially the older rocks. 158 ROCKS OF EARTH'S CRUST CHAP. (3) Rocks formed of the Remains of Plants or Animals. In Chapter VIII an account was given of the manner in which extensive accumulations are now being formed of the remains of plants and animals. Similar deposits have constantly been accumulated from an early period in the history of the earth. Regarding them with reference to their mode of origin, we observe that in some cases they have been piled up by the un- remitting growth and decay of organisms upon the same site. In a thick coral-reef, for example, the living corals now building on the surface are the descendants of those whose skeletons form the coral-rock underneath (p. 87). In other cases, the remains of the organisms are broken up and carried along by moving water, which deposits them elsewhere as a sediment. Strictly speaking, these last deposits are fragmental, and might be classed with those described at p. 148 ; they pass into ordinary sand, sandstone, clay, or shale. But it will be more convenient to class together all the rocks which consist mainly of organic remains, whether they have been directly built up by the 'organisms, or have only been formed out of their detrital remains. Limestone. As carbonate of lime is so largely secreted by animals in their hard parts which are more or less durable, it is naturally the most common substance among rocks of organic origin. The limestones that form so large a proportion of the stratified rocks of the earth's crust have been, for the most part, formed out of the remains of marine animals. The following are some of the more important or interesting varieties of this rock : Shell-marl, a soft white earthy crumbling deposit formed chiefly of fresh-water shells (pp. 4, 46); by subsequent infiltration it may be hardened into a compact stone, when it is known as fresh- water limestone ; Calcareous sand a mass of broken-up shells, calcareous algae, and other calcareous organisms (p. 84), often cemented by percolating water into solid stone ; Coral rock a limestone formed by the continuous growth of corals and cemented into a solid compact and even crystalline rock by the washing of calcareous mud into its interstices and the permeation of sea-water and rain-water through it, whereby crystalline calcite is deposited within it (p. 87) ; Chalk a soft, white rock, soiling the fingers, formed of a fine calcareous powder of remains of foraminifera, shells, etc. (see Ooze^ p. 86) ; Crinoidal limestone composed chiefly of the calcareous joints of the marine creatures known as crinoids, with foraminifera, shells, corals, and other xi ROCKS OF ORGANIC ORIGIN 159 organisms. A limestone composed in great part of organic remains may show little trace of its origin on a fresh fracture of the stone ; but a weathered surface will often reveal its true nature, the fossils being better able to withstand the action of the atmosphere than the surrounding matrix which is accordingly removed, leaving them standing out in relief (Fig. 76). FIG. 76. Weathered surface of crinoidal limestone. p ea t a yellow, brown, or black fibrous mass of compressed and somewhat altered vegetation. It occurs in boggy places in temperate latitudes where it largely consists of bog-mosses and other marshy plants (p. 82). Its upper parts are loose and full of the roots of living plants, while the bottom portions may be compact and black like clay, and with little trace of vegetable structure. Lignite or Brown Coal is a more compressed and chemically changed condition of vegetation. It varies in colour from yellow to deep brown or black, and may be regarded as an intermediate stage between peat and coal. It occurs in beds intercalated between layers of shale, clay, and sandstone. Coal a compact, brittle, black, or dark brown stone, formed of mineralised vegetation, and found in beds or seams usually resting on clay, and covered with sandstone, shale, etc. (see Figs. 79 and 140). There are many varieties of coal, differing from each other in the relative proportions of their constituents. Caking-coal, such as is ordinarily used in England, contains from 75 to 80 per cent of carbon, 5 or 6 per cent of hydrogen, and 10 160 ROCKS OF EARTH'S CRUST CHAP. or 1 2 per cent of oxygen, with some sulphur and other impurities. Anthracite, the most thoroughly mineralised condition of vegeta- tion, is a hard, brittle, lustrous substance, from which the hydrogen and oxygen have been in great measure driven away, leaving 90 per cent or more of carbon. Ironstone. Reference was made at pp. 47, 59, to ironstone precipitated from chemical solution. This precipitation is often caused through the medium of decomposing organic matter. Organic acids, produced by the decay of plants in marshy places and' shallow lakes, attack the salts of iron contained in the rocks or detritus of the bottom, and remove the iron in solution. On exposure, the iron oxidises and is thrown down as a yellow or brown precipitate of limonite or bog-iron-ore (p. 129), which is found in layers and concretions. Clay-ironstone, composed of a mixture of carbonate of iron, with clay and carbonaceous matter, occurs abundantly both as nodules and in layers, with remains of plants, shells, fishes, etc., in the Coal-measures (Figs. 61, 65, and bed 13 in Fig. 80), and has, no doubt, been also formed through the agency of organic acids which, passing into carbonic acid, have given rise to the solution and subsequent deposit of the iron as carbonate mingled with mud and with entombed plants and animals. Flint. Some siliceous deposits, due to organic agency, have been already referred to at p. 84. Besides these, mention may be made of Flint, which occurs as dark lumps and irregular nodular sheets in chalk and other limestones, frequently enclosing urchins, shells, and other organisms, which are sometimes con- verted into flint. Its mode of origin is not yet thoroughly understood, but there is reason to regard it as due to the abstraction of silica from sea -water, either directly, by such animals as sponges, or indirectly, by the decomposition of animal remains. Chert is a more impure siliceous aggregate found under similar conditions, especially among the older limestones. Guano a brown, light, powdery deposit, formed of the droppings of sea-birds in rainless tracts of the west coasts of South America and Africa. Containing much phosphate of lime as well as ammoniacal salts, it has great commercial value as an important manure. Bone-beds deposits composed of fragmentary or entire bones of fish, reptiles, or higher animals, as in the well-known bone-bed of the Rhastic series (p. 298). The floors of some caverns are covered with stalagmite, so full of pieces of the bones of cave- xi ERUPTIVE ROCKS 161 bears, hyaenas, and otaer extinct and living species, as to be called Bone-breccia. Layers of stone, full of the coprolites (fossil excrement) or of the rolled bones of various vertebrate animals, have, in recent years, been largely worked as sources of phosphate of lime for the manufacture of artificial manures. II. ERUPTIVE ROCKS. Under this division are grouped all the massive rocks which have been erupted from underneath into the crust or to the surface of the earth. They are composed chiefly of silicates of alumina, magnesia, lime, potash, and soda, with different propor- tions of free silica, ma'gnetic or other oxide of iron, and phosphate of lime. The principal silicate is generally some felspar, the number of eruptive rocks without felspar being comparatively small. The felspar is, in different rocks, conjoined with mica, hornblende, augite, magnetite, or other minerals. No perfectly satisfactory classification of the eruptive rocks has yet been devised ; they have been grouped according to their presumed mode of origin, some being classed as plutonic or hypogene^ from their supposed origin, deep within the earth's crust, others as volcanic^ from having been ejected by volcanoes. They have likewise been arranged according to their chemical composition, and also with reference to their internal structure. In the following enumeration of some of the more abundant and important varieties, it may be enough to adopt an arrangement in three sections, according to the nature of the predominant silicate: viz. (i) Orthoclase rocks; (2) Plagioclase rocks ; and (3) Olivine and Serpentine rocks. It has already been pointed out that the original condition of many lavas and other eruptive rocks has been that of molten glass, their present stony structure being due to the more or less complete devitrification and disappearance of the glass by the development of crystals and crystallites out of it during the process of cooling and consolidation (p. 144). Though there is no evidence that all crystalline eruptive rocks have once been in the state of molten glass, it may be useful to begin with the vitreous varieties, which we know to represent the earliest forms of many that are now quite crystalline. ( i ) Orthoclase Rocks. In this section the prevalent silicate is Orthoclase, either in its common dull, white, or pink form, or in the glassy condition M 162 ROCKS OF EARTH'S CRUST CHAP. (sanidine). In many of the rocks, free quartz occurs either in irregular crystalline blebs or in definite crystals, which frequently take the form of double pyramids. Among other minerals, horn- blende, white and black mica, and apatite are of common occur- rence. The rocks of this division are the most acid of the, eruptive series that is, they contain the largest proportion of silica or silicic acid, sometimes more than 75 per cent. Some of them (granite) are only found as masses that have consolidated deep beneath the surface ; others (trachyte, rhyolite, obsidian) are abundant as superficial volcanic products. Obsidian a black, brown, or greenish (sometimes yellow, blue, or red) glass, breaking with a shell -like or conchoidal fracture and into sharp splinters, which are translucent at the edges. Examined in a thin section under the microscope, the rock is found to owe its usual blackness to the presence of minute opaque crystallites (Fig. 69) which are crowded through it, not infrequently drawn out into streaky lines and curving round any larger crystal that may be embedded in the mass (Fig. 71 B). These arrangements, called flow-structure (p. 146), have evidently been caused by the movement of the rock while still in a fused state, the crystallites and other objects being borne onward by the currents of molten glass. In some obsidians, little spherulites of a dull grey enamel-like substance have made their appearance as stages in the devitrification of the rock (Fig. 71) ; but the mass has consolidated before the stony condition could be completed. In other instances, the whole rock has passed into a stony enamel-like mass with perlitic structure (pearlstom, p. 145). Where a still molten obsidian has been frothed up by the expansion of steam or gas through it, so as to become a spongy cellular substance which will float on water, it is called pumice. Obsidian occurs in many volcanic regions, sometimes as streams of lava which have been poured forth at the surface, sometimes in dykes and veins, and often in fragments ejected with the other detritus that now forms tuffs. Trachyte a compact porphyritic rock, consisting mainly of orthoclase (sanidine), with some plagioclase and usually with some hornblende, or with augite, mica., magnetite, or other minerals ; having a peculiar matrix whicn', under the microscope, is found to consist mainly of minute^ felspar-crystallites. Large crystals of orthoclase (sanidine) are frequent, and also scales of dark mica. This rock is found abundantly among some of the younger volcanic regions of the world, where it occurs in lava- xi ERUPTIVE ROCKS 163 streams and also in intrusive sheets and dykes. Quartz-trachyte {Liparite, Rhyolite] is a rock composed of a compact, often rough and somewhat porous base, through which are scattered crystals of felspar and blebs of quartz, often also with hornblende and mica. Felsite an exceedingly close-grained rock, composed of an intimate mixture of quartz and orthoclase. The felspar often occurs as large disseminated crystals, giving the porphyritic structure. Where the quartz appears as distinct blebs or crystals (sometimes double pyramids) the rock becomes Quartz-porphyry. The felsites and quartz-porphyries play an important part among the eruptive rocks of older geological time, occurring both in the form of lavas erupted to the surface and of intrusive masses that have consolidated below ground. Many of them can be proved to have been originally in the condition of molten glass which has been devitrified. Rocks which show the characteristic close- ness of grain characteristic of the felsites are said to \>efelsitic or to have a felsitic ground mass (p. 144). Syenite a thoroughly crystalline rock, consisting essentially of orthoclase and hornblende, and distinguished from granite chiefly by the absence or small amount of quartz. It occurs in bosses and veins which have been erupted into older rocks. Granite a thoroughly crystalline (holo-crystalline) compound of felspar, quartz, and mica, the individual minerals being large enough to be distinctly recognised by the naked eye. Sometimes large crystals of felspar are porphyritically scattered through the rock. Granite occurs in large eruptive masses which have been intruded into many different kinds of rocks, also in smaller bosses and veins. Round the outside of a mass of granite there frequently diverge from it dykes and veins (p. 203) which, where of great width, may show the usual granitic structure ; but which, when of small dimensions, are apt to appear as felsite or quartz- porphyry. There can be no doubt that such fine-grained veins are actually portions of the same mass of rock as the granite, so that granite and felsite or quartz-porphyry are only different conditions of the same substance, the differences being probably due to variations in the circumstances under which the cooling and consolidation took place. In the crystalline-granular struc- ture so distinctive of granite {granitic or granitoid, p. 144) the constituent minerals have not had room to assume perfect crystallised shapes, but occasionally they have been able to shoot out in perfect crystals where cavities occur. Fig. 77, for example, 1 64 . ROCKS OF EARTH'S CRUST CHAP. shows a group of the ordinary crystals of this rock which have crystallised in a cavity of the granite of the Mourne Mountains, Ireland. It is in such cavities also that the rarer minerals of this rock, such as topaz and beryl, may be looked for. FIG. 77. Group of crystals of felspar, quartz, and mica, from a cavity in the Mourne Mountain granite. (2) Plagiodase Rocks. In this section the felspar is some variety of plagioclase, and the other most frequent silicate is either augite or hornblende. Though free quartz occurs in some of the rocks, they contain generally so much less silica than the orthoclase rocks that instead of being acid they are commonly basic compounds. A range of texture can be observed in them similar to that characteristic of the orthoclase series, from a true glass up to a thoroughly crys- talline granitoid rock. Some of them, more especially the coarsely crystalline varieties, are probably of deep-seated origin ; others (and these include the great majority) are truly volcanic ejections which have risen in volcanic pipes and fissures, and have been poured forth at the surface as actual lava-streams. Basalt-Rocks a group of rocks consisting of plagioclase, augite, olivine, and magnetite or titaniferous iron, to which apatite and other minerals may be added. These rocks range in texture from a black glass up to a coarsely crystalline mass wherein the component minerals are distinctly visible to the naked eye. Different names are employed to distinguish these varieties. xi ERUPTIVE ROCKS 165 Basalt-glass ( Tachylyte, Hyalomelan) is a general epithet to denote the vitreous varieties. These are particularly to be observed along the edges of dykes and other intrusive masses, where they re- present the outer surface of the basalt that was suddenly chilled and consolidated by coming in contact with the cold walls of the vent or fissure into which it was injected, and where they no doubt show what was the original state of the whole basalt before devitrification converted the rock into its present crystalline structure (see pp. 97, 143). Basalt a black, compact, heavy, homogeneous rock, breaking with a conchoidal fracture, showing sometimes large porphyritic crystals of plagioclase, olivine, or augite, but too fine- grained for the component minerals of the base to be determined except with the microscope. The coarser varieties, where the minerals can be recognised with the naked eye, are known as Dolerite. The basalt -rocks are pre-eminently volcanic lavas, occurring both as intrusive masses that consolidated underground, and as sneets that were poured out in successive streams at the surface. The black, compact kinds (true basalt) are particularly prone to assume columnar forms (Fig. 78), whence columnar , rocks are sometimes spoken of as basaltic. In some varieties of basalt the mineral le_ucite takes the part of the plagioclase ; and in others this is done by another mineral, nepheline. Diabase a name given to some ancient basalt-rocks in which, owing to alteration of their augite or olivine, a greenish chloritic discoloration has often taken place. The lavas of early geological time are to a large extent diabase. Andesite is closely allied to basalt ; but contains no olivine. It sometimes includes free quartz, and hornblende may be sub- stituted in it for augite. Hornblende-andesite and Augite-andesite are lavas which have been extensively erupted in later geological time. Diorite a crystalline aggregate of plagioclase and hornblende, usually with magnetite and apatite, sometimes with augite and mica. The hornblende is black or dark green and often more or less decomposed, giving rise to a greenish chloritic discoloration of the felspar. From its prevalent green colour, the rock was formerly known as "greenstone." It occurs in intrusive masses, and seems generally if not always to have consolidated below ground instead of being poured out at the surface. Gabbro, Diallage-rock a thoroughly crystalline granitoid aggregate of plagioclase and the variety of augite known as diallage, which appears in distinct brown or greenish crystals, with 166 ROCKS OF EARTH'S CRUST CHAP. xi ERUPTIVE ROCKS SCHISTS 167 a peculiar metalloidal or pearly lustre ; it is found in bosses associated with granite, gneiss, etc., and also sometimes with volcanic rocks in centres of eruption. (3) Olivine and Serpentine Rocks. In this group may be included a comparatively small number of rocks which consist principally of olivine, and which by gradual alteration pass into serpentine (Fig. 58). Olivine-rocks (Perido- tites) are liable to remarkably rapid changes of texture and com- position. In some places they are mainly made up of olivine, augite, or hornblende, magnetite, and brown mica, but some of these minerals may disappear and some felspar may take their place. They are intrusive masses which appear to have been generally injected into the crust in connection with volcanic erup- tions, rather than to have been poured out at the surface in true lava-streams. Serpentine a compact, dull, or faintly glimmering rock, with a general dark dirty green colour, variously mottled, greasy to the touch, easily scratched, and giving a white powder which does not effervesce with acids. It is a massive form of the mineral ser- pentine described on p. 134; frequently containing disseminated crystals of the minerals bronzite, enstatite, and chromic iron, and veins of a delicately fibrous silky variety of serpentine known as chrysotile. Many serpentines were originally olivine-rocks which, by hydration and alteration of their magnesian silicates, have assumed their present characters. Serpentine occurs in bosses, dykes, and veins, which were evidently of eruptive origin and were at first probably olivine-rocks ; it is also found in thick beds associated with limestones and crystalline schists, where it may be a metamorphosed sedimentary rock. III. THE SCHISTS AND THEIR ACCOMPANIMENTS. This section includes a remarkable series of rocks of which the leading character is the possession of a schistose or foliated char- acter (Fig. 72). They are, in their more typical varieties, dis- tinctly crystalline. Some of them shade off into ordinary fragmental rocks, such as shale and sandstone ; others agree in chemical and mineral composition with some of the eruptive rocks already enumerated, into which they may often be traced by imperceptible gradations. In the schists, therefore, we see an assemblage of rocks which, 168 ROCKS OF EARTH'S CRUST CHAP. though possessing distinct characters of their own, may yet be observed to shade off into fragmental rocks on the one side, and into eruptive rocks on the other. In Chapter XIII some further account of them will be given, with special reference to their prob- able origin, and to the grounds on which they have been regarded as metamorphic or altered rocks. For the present, in taking notice of their composition and structure, it will be enough to state that in many cases they can be shown to be more or less altered and crystalline transformations of what were originally sedimentary rocks ; and that in other instances they represent original crystalline eruptive masses, which have been subjected to such enormous pressure and shearing, that a foliated structure and recrystallisation of minerals have been superinduced in them. The essential feature which unites masses of such different origin is the possession of that common schistose structure which they have derived from having all been alike subjected to the same kind of intense terrestrial movements. Clay-slate a hard fissile clay-rock, through which minute scales of mica and crystals or crystallites of other minerals have been developed ; generally bluish-grey to purple or green, and splitting into thin parallel leaves. As this rock often contains remains of marine animals and plants, and is interstratified with bands of sandstone, grit, conglomerate, and limestone, it was un- doubtedly at first in the condition of soft mud on the sea-bottom. Sometimes the organic remains in it are so curiously elongated or distorted in one general direction as to show that the rock has been drawn out by intense pressure and shearing (Figs. 98, 103, 104). The planes along which clay-slate splits are generally in- dependent of the original surfaces of deposit, sometimes cross these at a right angle, and have been superinduced by mechanical movements (Cleavage), as explained in Chapter XIII. Different varieties of clay-slate have received special names. Roofing slate is the fine compact durable kind, employed for roofing purposes and also for the manufacture of cisterns, chimneypieces, writing- slates ; Alum-slate dark, carbonaceous, and pyritous, the iron- disulphide oxidising into sulphuric acid, and giving rise to an efflorescence of alum ; Whet-slate honestone exceedingly hard, fine-grained, and suitable for making hones ; sometimes owing its hardness to the presence of microscopic crystals of garnet ; Chias- tolite-slate containing disseminated crystals of chiastolite, and found especially around eruptive bosses of granite. By increase of its mica-flakes a clay-slate passes into a Phyllite^ which has a xi SCHISTS 169 more silvery sheen, and represents a farther stage of metamor- phism. Phyllite, by increase of the mica, becomes Mica-slate, so that a transition may be traced from sedimentary fossiliferous rocks through clay-slate and phyllite into thoroughly crystalline schist. Clay-slate occurs extensively among the older geological formations in all parts of the world. Amphibolites rocks composed mainly of hornblende, but with quartz, orthoclase, and other minerals in minor proportions ; sometimes they are massive and granular (Hornblende-rock}, and in this condition doubtless represent eruptive rocks. Grada- tions can be followed from such rocks (originally diorite, diabase, etc.) into perfect schist (Hornblende-schist), so that the development of the schistose structure can be traced from rocks that were at first as structureless as any amorphous eruptive mass can be. Amphibolites occur among the crystalline schists in most parts of the world as occasional bands or bosses, which probably mark zones of basic igneous rock, either intruded into the accompanying masses, or contemporaneously erupted with them. Chlorite -schist a scaly, schistose aggregate of greenish chlorite with quartz, and often with felspar, mica, and octahedra of magnetite (Fig. 54); it occurs in beds associated with gneiss and other schists. Some chloritic schists may represent old lavas or other erupted rocks which have been crushed down and be- come schistose ; others, especially where they contain pebbles of quartz, etc., and are banded with quartzites and schistose con- glomerates, not improbably mark where fine volcanic ashes fell over a sea-bottom, and were then mingled and interstratified with the ordinary sediment that happened to be accumulating at the time. Mica-schist (Mica-slate) a schistose aggregate of quartz and mica, the two minerals being arranged in irregular but nearly parallel wavy folia. The rock splits along the laminae of mica, so that its flat surfaces have a bright silvery sheen, and the quartz is not well seen except on the cross fracture, where only the thin edges of the mica-plates present themselves. Mica-schist is often remarkably crumpled or puckered a structure bearing witness to the intense compression it has undergone (Fig. 1 14). It abounds in most regions where schists are extensively developed (Chapter XVI). Some mica -schists contain fossil shells and corals (Bergen), and must thus represent what were originally sediment- ary deposits ; others may be highly deformed eruptive rocks. Gneiss a schistose aggregate of orthoclase, quartz, and mica, 1 70 ROCKS OF EARTH'S CRUST CHAP, xi varying in texture from a fine-grained rock up to a coarse crystal- line mass which, in hand specimens, may not be distinguishable from granite. There is no difference indeed as regards composi- tion between gneiss and granite ; gneiss may be called a foliated granite. There is good reason to believe that some, if not all, true gneisses have been made out of granite or allied rocks by the process of shearing above referred to. Gneiss occurs abundantly among the oldest known rocks of the earth's crust, and may be found in most large regions of crystalline schists (Chapter XVI). A few rocks which are found associated with the schists, or with evidence of metamorphism, may be noticed here marble, quartzite, and schistose conglomerate. Marble a crystalline granular aggregate of calcite, white when pure, and having the texture of loaf-sugar, but passing into various colours according to the nature of the impurities. It occurs in beds among the schists, and is no doubt a limestone, formed either by chemical precipitation or by organic agency, which has been metamorphosed by heat and pressure into its present thoroughly crystalline character. Some of the fossiliferous limestones through which the Christiania granite rises have been changed into crystal- line marble, but their original corals and shells have not been wholly effaced (see Chapter XIV). Quartzite a hard, compact, granular rock, composed of adherent quartz-grains, and breaking with a characteristic lustrous fracture. It occurs in beds and thick masses, not infrequently associated with slates, mica-schists, and limestones ; it sometimes contains organic remains ; and is evidently an indurated siliceous sand. Schistose Grit and Conglomerate. Interstratified with clay- slates and mica-schists there are sometimes found beds of grit and conglomerate, the grains and pebbles of which consist of quartz or other durable material, imbedded in slate or schist. The original fragmental character of such rocks admits of no doubt ; they were obviously at one time sheets of fine and coarse gravel mixed with sandy mud ; and their presence among schistose rocks furnishes additional corroborative evidence of the original sedimentary character of some of these rocks. The clay or mud which formed the matrix has been metamorphosed into a more or less thoroughly crystalline micaceous substance, while in many cases the pebbles have been flattened and pulled out of shape. Hence these rocks afford important evidence as to the nature of the processes whereby the schists have been produced. PART III THE STRUCTURE OF THE CRUST OF THE EARTH CHAPTER XII SEDIMENTARY ROCKS THEIR ORIGINAL STRUCTURES HAVING in the two foregoing chapters considered the more important elementary substances of which the earth's crust is composed and their combinations in minerals and rocks, we have to inquire how these minerals and rocks have been 'put together so as to build up the crust. A very little examination will suffice to show us that the upper or outer parts of the solid globe consist chiefly of sedimentary rocks. All over the plains and low grounds of the earth's surface, which cover so large a proportion of the whole area of the land, some kind of sediment underlies the soil clay, sand, gravel, limestone. It is for the most part only in hilly or mountainous regions that anything has been pushed up from below, so as to indicate the nature of the materials underneath. But everywhere we encounter proofs that the sedimentary rocks do not remain as they were deposited. In the first place, most of them were laid down on the sea-floor, and they have been upraised into land. In the next place, not only have they been upheaved, they have not infrequently been bent, broken, and crushed, until sometimes their original condition can no longer be determined. Moreover they have been invaded by masses of lava and other eruptive rocks, which have been thrust in among them and have often burst through them to form volcanoes at the surface. We must now endeavour to* form as 172 STRUCTURES OF SEDIMENTARY ROCKS clear a conception as possible of what, after all these changes, the present structure of the crust actually is. In this chapter, therefore, we may examine some of the leading characters of sedimentary rocks in the architecture of the crust, more particu- larly those which have been determined by the conditions under which the rocks were formed. In the next chapter we shall consider some of the more important characters which have been superinduced upon the rocks since their formation. Stratification. It has been shown (p. 148) that one of the most distinctive features in sedimentary rocks is that they are stratified that is, are arranged in layers one above another. As those at the bottom must have been de- posited before those at the top, a succession of layers of stratified rocks forms a record of deposition, in which the early stages are chronicled by the lower, and the later stages by the upper layers. An illustration of this kind of record has already been given in the intro- ductory chapter. As a further ex- ample, the accompanying section (Fig. 79) may be taken. At the bottom lies a bed (a) of dark shale or clay with fragments of crinoids, corals, shells, and other marine organisms. Such a bed unmistakably points to a former muddy sea-floor, on which the creatures lived whose remains have been preserved in the hardened mud or shale. The next bed (b} is one of limestone full of similar organic remains ; it shows that the supply of mud, which had previously made the water turbid and had been slowly gathering in successive layers on the bottom, now ceased. The water became clear and much better fitted for the life of the crinoids, corals, and shells. These creatures accord- ingly flourished abundantly, living and dying on the spot generation after generation, until their accumulated remains had built up a solid sheet of limestone several feet thick. But once more muddy currents spread over the place, and from the cloud of suspended mud there slowly settled down the layer of blue clay (c} which overlies the limestone. As hardly any remains of organisms are to be seen in it, we may infer that the inroad of mud killed them off. Next, owing to some new shifting of the currents, a quantity FIG. 79. Section of stratified rocks. xii STRATIFICATION 173 of sand was brought in and spread . out over the mud, forming the sandstone beds (d). The sea in which these various strata were deposited was probably shallow ; or its floor may have been gradually rising. At all events, the last layers of sand could have been only slightly below the surface of the water, for they are immediately covered by a hardened silt or fire-clay (e) which, from the abundant roots and rootlets that run through it in all directions, was clearly once a soil whereon plants grew. It was probably part of a mud-flat, on which vegetation spread seaward from the land where the water shallowed, as happens at the present day among the tropical mangrove-swamps (p. 83). The plants that grew on this soil have formed the coal-seam (/), no doubt representing the growth of a long period of time. But the existence of the coal-jungle came to an end probably by a sinking of the ground beneath the water. Mud, once more carried hither from the neighbouring land, settled down upon the submerged vegetation and formed the clay (g). But that land plants still abounded in the immediate neighbourhood, is shown by their numerous remains in this clay. We notice too that the salts of iron dissolved in the water were eliminated by the decaying plants and animals and were precipitated in the form of carbonate, so as to form concretions round occasional dead shells, fishes, fern- fronds, and seed-cones. What were the immediately succeeding events in this ancient history we cannot tell ; the layer next in order is a coarse conglomerate (h\ originally gravel, which must have been swept along by a swift current that tore away the upper part of the clay-beds (g) and any strata which may once have overlain them. The whole stratified part of the earth's crust is composed of materials which in this way may be made to tell their story. In forcing them to yield up their records of the ancient changes of which they are memorials, scope is afforded for the most accurate and laborious investigation and for the closest reasoning from the facts collected. At the same time, it is obvious that the pursuit is one which constantly exercises the imagination, and that, indeed, it cannot be adequately followed unless, by the proper use of the imagination, the former conditions of the earth's surface are vividly realised. The thinnest layers of a stratified rock form lamina^ such as the thin paper-like leaves into which shale can be split. A number of laminae may be united in a stratum or bed which may vary from less than an inch to several feet or yards in thickness. It is only 174 STRUCTURES OF SEDIMENTARY ROCKS CHAP. i 1 .1 Li , l.i the finer kinds of sedimentary rock that, as a rule, are laminated. In other cases a stratum or bed is the thinnest subdivision ; it can usually be separated easily from those above and below it, and it may generally be regarded as marking one continued phase of deposit, while the break between it and the next bed above or below probably denotes an interruption of the deposit. The study of the relations of strata to each other is called Stratigraphy. Layers of deposit usually lie parallel with each other, their flat surfaces marking the general floor of the water at the time of their formation (Figs. 79, 80). But sometimes a series of layers may be found inclined at various angles to what was obviously the original general plane of deposition. In Fig. 8 1, for example, a series of strata is presented, which are distinguished by a diagonal lamination. This is known as False bedding or Current-bedding. As ex- plained in Chapter III (p. 37), it has been caused by the pushing of layers of sediment over the advancing front of a stratum, and may be compared to the oblique bedding often to be seen in an earthwork, such as a FtG.So.-Section showing railway embankment, the upper surface of alternation of beds. which may be in a general sense parallel 15. Shale. 14. Seam of with the flat bottom of the valley, while the sandstone. 13. Shale successive layers of which the mound is .rS: -^e are inclined at angles of 30' or more, stone. 10. Limestone. False bedding is interesting as affording 9. Clay. 8. Sandstones, some indication of the nature and direction 7 . Sandy clays. 6. Lime- o f tne curre nts by which sediment has been stone with parting of , , j shale. S .Shale P 4. Lime- transported. stone. 3 . Shale with Proofs of former Shores. Along the cement -stone passing margin of the sea, of lakes, and of rivers, down into sandstone (2), severa i interesting kinds of markings may which graduates into fine . . , , _ , conglomerate (x). be seen impressed on surfaces of sand or mud from which the water has retired. Every one who has walked on a tidal sea -beach is familiar TRACES OF SHORES AND LAND 175 with the Ripple-marks left by the retreating tide upon the bare sands. They are produced by the oscillation of the water driven into movement by wind playing over its surface. They are usually effaced by the next advancing tide ; hence, out of the same sand new sets of ripple-marks are made by each tide. But we can understand that now and then, under peculiarly favourable conditions, the markings may not be destroyed. If, for instance, they were made in a kind of muddy sand, which, in the interval between two tides and under a strong sun, could become hard and coherent on the surface, and if the next tide advanced so quietly as not to disturb them, but to lay down upon them a fresh layer FIG. 81. False-bedded sandstone. of sand or mud, they might be covered up and preserved. They would then remain as a memorial of the shallow rippling water and bare sandy shore where they had been formed. Now evidence of this kind regarding* the conditions of deposi- tion occur abundantly among sedimentary rocks (Fig. 82). Ripple-marked surfaces may be traced one over another for many hundred feet in a thick series of sandstones. They bring clearly to the mind that the strata on which they lie were accumulated in shallow water, or along beaches that were often laid dry. Land-Surfaces. Other traces of exposure to the air may be noticed, where ripple-mark is abundant, in what are termed Sun- cracks^ Foot-prints^ and Rain-prints. Those who have observed what takes place in muddy places during dry weather will 176 . STRUCTURES OF SEDIMENTARY ROCKS CHAP. remember that, as the mud dries and contracts it splits up into a network of cracks ; and that, on its hardened surface, it retains impressions of the feet of birds or of insects that may have walked over it while still soft. The geological history recorded at such places cannot be mistaken ; first, the rainy period, with the rush of muddy water down the slopes and the formation of pools in which the mud is allowed to settle ; then the season of warm weather when the pools gradually dry up and birds seek their edges to drink. If by any means a layer of sediment could be FIG. 82. Ripple-marked surface of sandstone. laid down upon one of these desiccated basins so gently as not to efface its peculiar markings, the cracked surface of mud, with its footprints, would contain a perfectly intelligible record of the changes which it had witnessed (Fig. 83). Now surfaces of this kind abound among the sedimentary rocks of the earth's crust. They are found upon strata which, from the presence of marine organic remains in them, were certainly deposited under the sea. But these strata cannot have accumu- lated in deep water ; they must have been formed along flat' shores, where the sheets of sand and mud were liable from time to time to be laid bare to the sun and wind, where animals of various kinds left their footmarks or trails on the still soft xii RAIN-PRINTSCONCRETIONS 177 sediment, where the evaporation and desiccation were so rapid as to cause the exposed mud to harden on the surface and to crack up into irregular polygonal cakes, and where the next succeeding layers of sediment were deposited so gently as to cover up and preserve the sun-cracked surfaces. One further piece of evidence to indicate land-surfaces, or, at least, shore-surfaces, in a series of aqueous sedimentary strata, is that furnished by Rain-prints. A brief shower of rain leaves upon a smooth surface of fine sand or mud a series of small pits, FIG. 83. Cast of a sun-cracked surface preserved in the next succeeding layer of sediment. . each of which is the imprint of a descending raindrop (Fig. 84). Where this takes place along the edge of a muddy pool which is rapidly being dried up, the prints of the drops may remain quite distinct on the hardened surface of mud. And here, again, we can suppose that if another layer of mud were gently deposited above this surface the rain-prints would be sealed up and preserved. We might even be able to tell from what quarter the wind blew that brought the rain-cloud. If, for example, the rain-prints were ridged up on one side in one general direction this would show that the shower fell aslant and with some force, and that the side on which the mud round the imprints was forced up was that towards which the rain was driven. Such indications of ancient weather may here and there be detected among stratified rocks. Concretions. Another original characteristic of many sedi- mentary rocks is a concretionary structure, particularly observable Iff 1 78 STRUCTURES OF SEDIMENTARY ROCKS CHAP. in clays, limestones, and ironstones. In many cases, the con- cretions have gathered round some fragment of a plant or an animal. Clay-ironstone and impure limestone have been aggre- gated into spherical or elliptical forms (septaria), which are of frequent occurrence in clay or shale (Figs. 61, 65). Flint has also gathered round some organic nucleus, which it has often entirely replaced. But many concretions may be found where no organic fragment as a starting-point can be detected. Some of the most curious are the so-called Fairy-stones (Fig. 64), found in alluvial clays, with so many imitative shapes, which have been FIG. 84. Rain-prints on fine mud. popularly supposed to be works of human or even preternatural construction. They have probably been produced by the irregular cementing of clay, owing to the spread of carbonate of lime through it, carried down by permeating water. Some of the most extraordinary concretionary masses are to be seen in certain magnesian limestones, which appear to be built up of petrified lumps of coral, bunches of grapes, cannon-balls, and other objects (Fig. 75). In reality, all these diversified figures are due to the irregularly varied way in which a concretionary structure has been developed in the limestone. Association and Alternation of Strata. Certain kinds of sedimentary rocks are apt to occur together to the exclusion of others. This association depends on the circumstances of xii DISPOSITION OF MECHANICAL SEDIMENT 179 deposition. Ironstone concretions, for example, are much more frequent among clays or shales than in any other strata, because it was during the deposit of fine mud with abundant decomposing organic matter that the most favourable conditions were supplied for the precipitation of carbonate of iron. Clays and limestones frequently alternate, as also do sandstones and conglomerates, because the circumstances of deposition were somewhat alike (see Fig. 80). But we need not expect to encounter a bed of coarse conglomerate in a group of fine clays, for the current that was strong enough to sweep along the stones of the conglomerate was too powerful to allow the fine silt to lie undis- turbed. For a similar reason, we should be surprised to meet with a layer of well-stratified shale in a mass of conglomerate. The agitated water in which these coarse materials were heaped up would have swept away any fine sediment and prevented it from being deposited. In all cases, the manner in which the different kinds of sediment are associated with each other leads us back directly to the original conditions of deposit, and is only intelligible in proportion as these conditions are clearly realised. Relative Areas of Stratified Eocks. Moreover, some kinds of sedimentary material must obviously spread over wider areas than others. The coarse gravel and shingle of the present beach do not extend far seawards ; they are confined to the margin of the land. Sand covers the sea -floor over a wider area ; and beyond the limits of the sand, in the deeper and stiller water, mud is allowed to accumulate. Roughly speaking, therefore, the area of the distribution of sediment is in inverse proportion to the coarseness of the materials. The same law has regulated the accumulation of detritus from early geological time. Coarse conglomerates, which represent ancient shingles and gravels, thicken and thin out rapidly, and do not usually cover a large area, though they may sometimes be traced for long distances in the direction probably of the original coast or line of heaping up of the shingle. They pass laterally and vertically into grits and sandstones which have a much wider distribution, and these again shade off into clays and shales that range also over large areas. Chronological Value of Strata. No clue has yet been found to determine the length of time required for the accumulation of a stratum or group of strata ; but some indications are afforded of relative lapse of time. Here and there, for instance, vertical trunks of trees are met with standing in their positions of growth, i8o STRUCTURES OF SEDIMENTARY ROCKS CHAP. but imbedded in solid sandstone (Fig 85). These stems, some- times 20 feet or more in height, prove that a mass of sand of that depth must have been accumulated around them before they had time to decay. We know little about the durability of the submerged trees ; but they probably could not have lasted long unless covered up by sediment ; so that the mass of strata in which they are enclosed may be supposed to have been accumu- lated within a few years. The nature of the material composing FIG. 85. Vertical trees (Sigillarid) in sandstone, Swansea (Logan). sedimentary rocks may likewise furnish indications of relative rate of deposition. Thus finely laminated clays were evidently deposited with extreme slowness. Beds of limestone, composed of the crowded remains of successive generations of marine creatures, must also have required prolonged periods of time for their growth. On the other hand, thick beds of sandstone pre- senting great uniformity of characters may not improbably have been laid down with comparative rapidity. No reliable inference can be drawn from the mere thicknesses of strata as to the lapse of time which they represent. A mass xii CONDITIONS OF SEDIMENTATION 181 of sandstone 20 feet thick may have accumulated round a sub- merged tree in a few years. On the other hand, a corresponding depth of fine laminated clay may have required tenfold more time for its deposition. But the same thickness of rock composed of alternations of shale and limestone might represent a still longer period. For it is obvious that the change from one kind of sediment to another must often have been brought about by an extremely gradual modification of the geography of the region from which the supply of sediment was derived. Hence the interval between two beds or groups of beds, differing much from each other in mineral composition, may have been considerably longer than the time required for the actual deposition of the strata of either or both beds or groups of beds. On any probable estimate, the deposition of sedimentary rocks to a depth of many thousand feet and over areas many thousands of square miles in extent, must have demanded enormous periods of time. Side by side with the growth of mechanical sediments, there must have been a corresponding wasting of land. Every bed of conglomerate, sand, or mud represents at least an equivalent amount of rock worn away from the land and trans- ported as sediment to the floor of the sea. During such prolonged ages as these changes required, there was ample time for the outburst of many successive volcanoes, for the passage of many earthquake-shocks, and for the subsidence or upheaval of many parts of the earth's crust. Proofs of Subsidence. A mass of sedimentary material of FIG. 86. Hills formed out of horizontal sedimentary rocks. great thickness which, from the remains of sun-cracks and other evidence, was obviously deposited in shallow water near land can only have been accumulated on an area that was gradually sink- 182 STRUCTURES OF SEDIMENTARY ROCKS CHAP. ing. Suppose, for instance, that a hill formed out of such strata rises a thousand feet above the valley at its foot (Fig. 86), and that proofs of deposition in shallow water can be detected from the lowest beds all the way up to the highest. The lowest beds having once been close to the surface, as shown by the sun-cracks and other evidence, could only be covered with hundreds of feet of similar strata by a gradual sinking of the ground, during which fresh sediment was poured in, so that, although the original bottom sank a thousand feet, the water may never have become sensibly deeper, the rate of deposit of sediment having, on the whole, kept pace with that of the subsidence. Overlap. During such tranquil movements, as the area of land lessens and that of the sea increases, the later sedimentary accumu- lations must needs extend beyond the limits of the older ones. Suppose, for instance, that such a sloping land-surface as that FIG. 87. Section of overlap. represented in the section (.r, Fig 87) were slowly to subside beneath the sea, the first -formed strata (a) will be covered and overlapped by the next series (b\ and these in turn, as the sea- floor sinks, will be similarly concealed by the following group (c). This structure, termed Overlap, may usually be regarded as evidence of a gentle subsidence of the area of deposit. Conformability, Unconformability. When stratified deposits are laid down regularly and continuously upon each other, with no interruption of their generally level position, they are said to be conformable. In the section Fig. 80, for instance, the series of sediments there represented has evidently been deposited under the same general conditions. The nature of the sediment has of course varied from time to time ; limestones, shales, and sandstones have alternated with each other ; but there has been no marked interruption or disturbance in their sequence. Suppose, however, that owing to subterranean movements, a series of rocks (a in Fig. 88) is shifted from its original position, and after being uplifted, is exposed to the wearing action of the sea, rivers, air, rain, frosts, and the other agents concerned in the degradation of xii UNCONFORMABILITY 183 the surface of the land. If a new series of deposits (b) is laid down upon the denuded edges of these rocks, the bedding of the whole will not be continuous. The younger strata will rest successively upon different parts of the older group, or, in other words, will be unconformable. Such a relation or unconformability (unconformity) implies a terrestrial disturbance, and usually also the lapse of a long interval of time between the respective periods of the older and younger rocks, during which denudation of the Dlder strata took place. It serves to mark one of the breaks or gaps in geological history. Unconformabilities differ much from each other in regard to the length of interval which they denote. In some cases, the blank may be of comparatively slight moment ; in others, it is so vast as to include the greater part of the time represented by the stratified rocks of the earth's crust. FIG. 88. Unconformability. By means of Unconformabilities the different ages of mountain- chains are determined. If, for example, a mountain showed the structure represented in Fig. 88, its upheaval must obviously have taken place between the deposition of the two series of rocks. Suppose the series a to represent Lower Silurian, and b Carboni- ferous rocks, the date of the mountain would be between the Lower Silurian and Carboniferous periods. If, in another mountain, series b were unconformably overlain by a younger series, say of Jurassic age, this mountain would thereby be shown to have undergone a subsequent uplift in the long interval between the Carboniferous and the Jurassic periods. Summary. In this Lesson some of the more characteristic original features of sedimentary rocks have been considered. Of these features, one of the most distinctive is the arrangement into layers of beds, each of which is the record of a portion of geologi- cal history, the oldest being below and the youngest above. The smallest subdivision of these records is a lamina or thin leaf, such as those into which shales may be split. A stratum or bed, 184 STRUCTURES OF SEDIMENTARY ROCKS CHAP, xn which may contain many laminae or none, is a thicker layer separable with more or less ease from those below and above it. Though strata lie on the whole parallel with each other, they often show oblique current-bedding, especially in sandstones. Traces of shore-lines and of surfaces laid bare by the retirement of the water in which they were deposited, are found in sun-cracks, rain- pittings, and footprints. Not infrequently, instead of being evenly spread out in layers, the sedimentary material has been aggregated into variously-shaped concretions. Certain kinds of sedimentary rocks are apt to occur together, such as clays and limestones, clay-ironstones and shales, coals and fire-clays ; because the con- ditions under which they were respectively deposited were on the whole similar. As a rule, the finer the detritus, the wider the area over which it is spread ; hence clays generally cover wider tracts than conglomerates. No inference can safely be drawn from the relative thickness of strata as to the length of time which they respectively represent ; they must vary widely in this respect, and it is quite conceivable that, in many cases, the interval of time between the deposition of two successive beds of very different character and composition may have been actually longer than the period required for the deposition of the two beds. A thick series of sedimentary deposits usually indicates that the sea- bottom on which it was laid down was slowly sinking. In sub- siding, the later deposits spread beyond the limits of the earlier ones, and thus present what is called an overlap. Where they have been laid down continuously one upon another they are said to be conformable ; where one group has been deposited on the disturbed and worn edges of an older series the two are un- conformable to each other. CHAPTER XIII SEDIMENTARY ROCKS STRUCTURES SUPERINDUCED IN THEM AFTER THEIR FORMATION AFTER their deposition sedimentary materials have undergone various changes before assuming the aspect which they now wear. Consolidation. The most obvious of these changes is that, instead of consisting of loose materials, gravel, sand, mud, and so on, they are now hard stone. This consolidation has sometimes been the result of mere pressure. As bed was piled over bed, those at the bottom would gradually be more and more compressed by the increasing weight of those that were laid down upon them, the water would be squeezed out, and any tendency which the particles might have to cohere would promote the consolidation of the mass. Mud, for example, might in this way be converted into clay, and clay in turn might be pressed into mudstone or shale. But besides cohesion from the pressure of overlying masses, sedimentary matter has often been bound together by some kind of cement, either originally deposited with it or subse- quently introduced by permeating water. Among natural cements, the most common are silica, carbonate of lime, and peroxide of iron. In a red sandstone, for example, the quartz-grains may be observed to be coated over with earthy iron peroxide, which serves to unite them together into a more or less coherent stone. The effect of weathering is not infrequently to remove the binding cement, and thereby to allow the stone to return to its original condition of loose sediment. Joints. Next to their consolidation into stone, the most com- mon change which has affected sedimentary rocks is the production in them of a series of divisional planes or fractures termed Joints. Except in loose incoherent materials, this structure is hardly ever absent. In any ordinary quarry of sandstone ? limestone, or other 186 STRUCTURES OF SEDIMENTARY ROCKS CHAP. sedimentary rock, or along a natural cliff of the same materials, a little attentive observation will show that the bare wall of rock forming the back of the quarry or the face of the cliff has been determined by one or more natural fissures in the stone, and that there are other fissures running parallel with it through every outstanding buttress of rock. Moreover, we may ob- serve that these vertical or highly inclined lines of fissure are cut across by others, more or less nearly at a right angle, and that the sides of the buttresses have been defined by these trans- verse lines, just as the main face of rock has been formed by the FIG. 89. Joints in a stratified rock. first set. Such lines of division are Joints. In close-grained stone they may be imperceptible until it is quarried or broken, when they reveal themselves as sharply defined, nearly vertical fractures, along which the stone splits. There are usually at least two series of joints crossing each other at right angles or obliquely, whereby a rock is divided into quadrangular blocks. In the accompanying diagram (Fig. 89) a group of stratified rocks is seen to be traversed by two sets of joints, one of which (dip- joints, " cutters " of the quarrymen) defines the faces that are in shadow, the other (strike-joints, " backs " of the quarrymen) those that are in light. By help of these divisional planes, it is possible to obtain large blocks of stone for building purposes. The art of the quarryman, indeed, largely consists in taking advantage of these natural lines of fracture, so as to obtain his materials with the least expenditure of time and labour, and in large masses. xin JOINTS DIP 187 In nature also the existence of joints is a fact of the highest im- portance. Reference has already been made to the way in which they afford a passage for the descent of water from the surface. It is in great measure along joints that the underground circula- tion of water is conducted. At the surface, too, where rocks yield to the decomposing influence of the weather, it is by their joints that they are chiefly split up. Along these convenient planes of division, rain-water trickles and freezes ; the walls of the joints are separated, and the space between them i$ slowly widened, until in the end it opens into yawning rents, and portions of a cliff are overbalanced and fall, while detached pinnacles are here and there isolated. The picturesqueness of the scenery of stratified rock is, in great measure, dependent upon the influence of joints in promot- ing their dislocation and disintegration by air, rain, and frost. In many cases, joints may be due to contraction. A mass of sand or mud, as it loses water and as its particles are more firmly united to each other, gradually occupies less room than at first. In consequence of the contraction strains are set up in the stone, and relief from these is eventually found in a system of cracks or fissures. In other instances, joints have been produced by the compression or torsion to which large masses of rock have been exposed during movements of the earth's crust. Original Horizontality. As laid down upon the margin or floor of the sea, on the bottoms of lakes, and on the beds or alluvial plains of rivers, sedimentary accumulations are in general nearly flat. They slope gently, indeed, seawards from a shelving shore, and they gather at steeper angles on slopes of debris at the foot of cliffs, or down the sides of mountains. But, taken as a whole, and over wide areas, their original position is not far re- moved from the horizontal. If we turn, however, to the sediment- ary rocks that form so large a part of the earth's crust, and so much of the dry land, we find that although originally deposited for the most part over the sea-bottom, they are now inclined at all angles, and even sometimes stand on end. Such situations, in which their deposition could never have taken place, show that they have been disturbed. Not only have they been upraised into land, but they have been tilted unequally, some parts rising or sinking much more than others. Dip. The inclination of bedded rocks from the horizon is called their Dip. The amount of dip is reckoned from the plane of the horizon. A face of rock standing up vertically above that plane is said to be at 90, while midway between that position and i88 STRUCTURES OF SEDIMENTARY ROCKS CHAP. horizontally it lies at an inclination of 45. The angle of dip is accurately measured with an instrument called a Clinometer, of which there are various forms. One of the simplest kinds is a brass half-circle graduated into 90 on each side of the vertical, on which a pendulum is hung as in Fig. 91. The instrument is held between the eye and the angle to be measured, and the upper FIG. 90. Dip and Strike. The arrow shows the direction of dip ; the line 5 s marks the strike. edge is made to coincide with the line of the inclined rock. The pendulum, remaining vertical, points to the angle of inclination from the horizon. A little practice, however, enables an observer to estimate the amount of dip by the eye with sufficient accuracy for most purposes. The direction of dip is the point of the com- pass toward which a stratum is inclined (shown by the arrow in Fig. 90), and is best ascertained with a magnetic compass. But FIG. 91. Clinometer. here again a little experience in judging of the quarters of the sky without an instrument will usually enable us to tell the direc- tion of dip with as much precision as may be required. Strike. A mathematical line running at a right angle to the direction of dip is called the Strike (s s in Figs. 90, 92). Where a series of strata dips due north or due south the strike is east and west ; but the direction of strike changes with that of the dip. xiii STRIKE OUTCROP 189 Suppose, for example, that certain strata dip due east, then veer round by south-east to south, and so on by west and north, back to east again. The strike following this change would describe a circle. In fact, the beds would be included in a basin-shaped or dome-shaped arrangement and the strike would be the lip of the basin or rim of the truncated dome. Though the dip may slightly vary from place to place, still, if it remains in the same general direction along the line of certain strata, their strike is on the whole uniform. Outcrop. The actual edge presented by a stratum at 'the surface of the ground is called its Outcrop. On a perfectly level FIG. 92. Dip, Strike, and Outcrop. surface, strike and outcrop must coincide ; but as ground is seldom quite level they usually diverge from each other, and do so the more in proportion to the lowness of angle of dip and the inequali- ties of the ground. This may be illustrated by a diagram such as that given in Fig. 92, which represents a portion of the edge of a table-land, deeply trenched by two valleys that discharge their' waters into the plain below (P). The arrows point out that the strata dip due N. at 5. On the level plain, the outcrop and the strike (s s) of the beds are coincident and run due E. and W. But as the surface rises towards the high ground and the deep valleys, the outcrop (o o) is observed to depart more and more from the strike till in some places the two lines are at right angles ; yet, as the dip remains the same, the strike is likewise unchanged, igo STRUCTURES OF SEDIMENTARY ROCKS CHAP. the sinuosities of the outcrop being entirely due to the irregulari- ties of the surface of the ground. Curvature. It requires no long observation to perceive that in being tilted from their original more or less level positions, stratified rocks have been thrown into curves. Suppose, for instance, that in walking along a mile of coast-line, where all the successive strata of a thick series are exposed to view, we should / /* / / FIG. 93. Inclined strata shown to be parts of curves. observe such a section as is drawn in Fig. 93. Beginning at A, we find the beds tilted up at angles of 70 which gradually lessen, till at B they have sunk to 15. As there is no break in the series, it is evident that the lines of bedding must be prolonged downward, and must once have been continued upward in some such way as is expressed by the dotted lines. The visible portion which is here shaded must thus form part of a great curvature of the rocks. But the actual curvature may often be seen on coast-cliffs, ravines, or hillsides. In Fig. 94, for example, a simple arch is XIII CURVATURE, PLICATION, SHEARING 191 shown from the Berwickshire coast, wherein hard beds of grey- wacke and shale have been folded. Again, in Fig. 95, the reverse structure is exhibited, beds of grit and slate being there curved into a trough. Where rocks dip away from a central line of axis the structure is known as an Anticline ; where, on the other hand, they dip towards an axis it is called a Syncline. In Figs. 94 and 95 these two structures are presented on so small a scale as to be visible in a single section. More usually, however, it is only by FIG. 94. Curved strata (anticlinal fold), near St. Abb's Head. observing the upturned edges of strata that anticlines and synclines can be detected. The dark part of Fig. 96 represents all that can be actually seen ; but the angles and direction of dip leave no doubt that if we could restore the amount of rock which has here been worn away from the surface of the land, the present truncated ends of the strata would be prolonged upward in some such way as is indicated by the dotted lines. By observations of this truncation of strata some of the most interesting and important evidence is obtained of the enormous extent to which the land has been reduced by the removal of solid material from its surface. Plication, Shearing. From such simple curvatures as those 192 STRUCTURES OF SEDIMENTARY ROCKS CHAP depicted in the foregoing diagrams, we may advance to more complex foldings, wherein the solid strata have been doubled up and crumpled together, as if they had been mere layers of carpet. So far is this plication sometimes carried, that the lowest rocks are brought up and thrown over the highest, the more yielding materials being squeezed into the most intricate frillings and puckerings. It is in mountainous regions, where the crust of the earth has been subjected to the most intense corrugation, that FIG. 95. Curved strata (synclinal fold), near Banff. these structures are best seen. We can form some idea of the gigantic energy of the earth-movements that produced them, when we see a whole mountain-range made up of solid limestones or sandstones which have been bent, twisted, crumpled, and inverted, as we might crush up sheets of paper (Fig. 97). So enormous has been the compression produced by important movements of the earth's crust, that the solid rocks have actually been squeezed out of shape or have undergone a process of shearing. The amount of distortion may sometimes be measured by the extent to which shells or other organic remains are pulled out in the direction of movement. In Fig. 98 the proper shape of a XIII CLEAVAGE 193 trilobite (Angelina SedgwickH) is given, and alongside of it is a view of the same organism which has been elongated by the dis- tortion of the mass of rock in which it lies. Further results of FIG. 96. Anticlines (a a) and Synclines (b V). shearing will be immediately referred to in connection with the cleavage and metamorphism of rocks. Cleavage. One of the most important structures developed by the great compression to which the rocks of the earth's crust have been exposed is known as Cleavage. The minute particles of rocks, being usually of irregular shapes, have been compelled to arrange themselves with their long axes perpendicular to the direction of pressure during the interstitial movements consequent FIG. 97. Section of folded and crumpled strata forming the Grosse Windgalle (10,482 feet), Canton Uri, Switzerland, showing crumpled and inverted strata (after Heim). upon intense subterranean compression. Hence, a fissile tendency has been imparted to a rock, which will now split into leaves along the planes of rearrangement of the particles. This super- induced tendency to split into parallel leaves, irrespective of what may have been the original structure of the rock, constitutes cleavage. It is well developed in ordinary roofing-slate. Though O 194 STRUCTURES OF SEDIMENTARY ROCKS CHAP. the leaves or plates into which a slate splits resemble those in a shale, they have no necessary relation to the layers of deposition but may cross them at any angle. In Fig. 99, for instance, the original bedding is quite distinct and shows that the strata have been folded by a force acting from the right and left of the section ; the parallel highly inclined lines traversing the folds of the bedding represent the planes of cleavage. Where the material is of ex- ceedingly fine grain, such as fine consolidated mud, the original bedding may be entirely effaced by the cleavage, and the rock will only split along the cleavage-planes. Indeed, the finer the grain of a rock, the more perfect may be its cleavage, so that where alternations of coarser and finer sediment have been sub- FIG. 98. Distortion of fossils by the shearing of rocks ; () a Trilobite (Angelina Sedg- wickii) distorted by shearing, the direction of movement indicated by the arrows ; (b) the same fossil in its natural form. jected to the same amount of compression, cleavage may be perfect in the one and rudely developed in the other, as is indicated in Fig. 99. Cleavage may be regarded as one of the first stages in the mechanical deformation of a rock, and the production of schistose metamorphism (p. 167). Besides being compressed and having its component particles rearranged in definite planes, the rock may likewise reveal under the microscope that new minerals, such for example as crystallites or minute flakes of some mica, have been developed out of the general matrix, as may be seen in common roofing-slate. By increasing stages of crystallisation we trace gradations into phyllites and mica-schists. Dislocation. Another important structure produced in rocks after their formation is Dislocation. Not only have they been folded by the great movements to which the crust of the earth has been subjected, but the strain upon them has often been so great that they have snapped across. Such ruptures of continuity pre- DISLOCATIONS 195 sent an infinite variety in the position of the rocks on the two sides. Sometimes a mere fissure has been caused, the rocks being simply cracked across, but remaining otherwise unchanged in their relative FIG. 99. Curved and cleaved rocks. Coast of Wigtonshire. The fine parallel oblique lines indicate the cleavage, which is finer in the dark shales and coarser in the thicker sandy beds. situations. But, in the great majority of instances, one or both of the walls of a fissure have moved, producing what is termed a Fault. Where the displacement has been small, a fault may appear as if the strata had been sharply sliced through, shifted, FIG. ioo. Examples of normal Faults. and firmly pressed together again (a in Fig. ioo). Usually, how- ever, they have not only been cut, but bent or crushed on one or both sides () ; while not infrequently the line of fracture is repre- sented by a band of broken and crushed material (Fault-rock^ c). The fracture is seldom quite vertical ; almost always it is inclined 196 STRUCTURES OF SEDIMENTARY ROCKS CHAP. at angles varying up to 70 or more from the vertical. In by far the largest number of faults, the inclination of the plane of the fissure, or what is called the Hade of the fault, is away from the side which has risen or toward that which has sunk. In the ex- amples given in Fig. I oo, a, b, this relation is expressed ; but in nature it often happens that the beds on two sides of a fault are entirely different (Fig. 100,